I was there in May, and was unable to find anywhere offering SIM cards. I ended up renting a local CDMA handset.

You won't, and will never, be able to use that handset in Korea. They simply don't have 2G GSM services.

Additionally, my understanding is that given the current law, you'd never be able to buy a SIM and just put it into a new (unlocked, 3G) handset in any case. You need to buy a service plan and register your phone with the authorities. Only whitelisted phones can be used in Korea with Korean plans.

So, your choices are:
- Buy a handset (can probably get a so-called "card phone" second hand from Itaewon)
- Rent a handset with its own service plan and pay the bill when you leave Korea
- Rent a handset that takes your own SIM, and roam
- Buy a 3G handset (from home) and roam

Basically what I suspected. Thanks for you explanations, highly appreciated!

Some of the roaming SIMs have free incoming calls in Korea. That might be possibility for some.

rental is the way to go.

you can usually find 50% off daily rental rate on random sites for s-roaming. you can even get the number assigned to you before you land in korea. incoming calls are free, and rates are reasonable since they do not bill by the minute.

South Korea and Japan have GSM 3G networks on 2100MHz BUT NOT GSM 2G networks.

As long as you have a STANDARD GSM-3G phone with the 2100MHz band used in Europe & Asia it will work fine whether roaming or if you can buy a SK Telecom or the other telecom carrier's GSM 3G SIM card.

In Japan this is NTT DoCoMo and Vodafone Japan (or the local name I forgot).

They ONLY have CDMA, 2 varients: CDMA2000 1X and EVDOrA and GSM-3G WCDMA/UMTS/HSPA+.

If you're from Europe your EU handset as long as it has 3G is good to go, just get it unlocked, it uses the same band.

If you're from the Americas make sure you have a GSM 3G phone with the 2100MHz band (same as Europe) and all the newer AT&T and T-Mobile phones HAVE THIS BAND.

If you have Verizon/Sprint there is also CDMA2000 1X/EVDO roaming there as long as you have a updated PRL (dial *228, option 2 on Verizon, Sprint is a different sequence) on the native network and you can roam there on the CDMA2000.

There is NO CDMA2000 CSIM card outside of those countries. If you're foreign you roam on the GSM 3G network (2100MHz UMTS/HSPA+, aka AT&T-TMobile technology).

AND why would you NOT want a GSM 3G phone, it allows access of 3 to 5x the amount of voice/data capacity.

Japan/South Korea are 1st world Asian countries, they were actually smart in getting rid of GSM 2G/EDGE very crappy spectral efficient technology.

As already noted, there is no direct way to do this, unfortunately!

They didn't "get rid" of them - they never had them. Korea was an IS-95 country while Japan used its own technology, PDC (which was also TDMA, like GSM...)

they may offer rentals on wi-bro (i.e. wimax) enabled phones - the carriers were thinking about doing it about a year a go so not sure whether they ended up following through.

Limited Korean SIM Card Offer Aug 12, 2011 ~ Sep 10, 2011
 

For the limited time you can purchase Korean SIM card between Aug 12, 2011 ~ Sep 10, 2011. I haven't read all of T&C but check it out at
http://www.sroaming.com/eng/Product/simcard.asp

SIM cards for Korea/Japan
 

You can get a hybrid phone (UMTS/GSM) that also has a slot for a SIM card. The European version of the Nokia 1280 is an example http://www.cellularabroad.com/japan-phone.php. Then, you can get an international roaming SIM card for that phone that works in Korea and Japan. As far as getting a country specific SIM card for Korea or Japan, no, they don't exist.

With regards to Japan:

Softbank sell prepaid handsets but they don't sell prepaid SIM cards on their own.
The phones run to about 7000 Yen (this includes 3000 yen calling credit) I wouldn't advise taking the SIM from the prepaid phone and putting it into your own phone, as although things like calling and SMS will work, MMS is slightly different (email address is assigned to your phone, can't use a phone number to MMS someone on another network etc.) Since July you can now SMS other networks using just their phone number, but most people don't know that this exists yet (they "mail" i.e. MMS rather than "text" being SMS, confusing I know.)

If you're only going to be in Japan for a short time, you might want to consider renting a SIM or a handset.

I've used Softbank before and they were pretty good. Link is http://www.softbank-rental.jp/en/
I believe that DoCoMo also do rental services at the airport (and probably au do as well) but I can't comment on the quality of service.

1 world 1 number
 

Just visit http://travel-gsm.com ^

It's possible to get a prepaid GSM SIM card in South Korea as of the beginning of this year. Since South Korea never had a 2G GSM network, the SIM card will only work in phones with 3G WCDMA 2100MHz support (most 3G phones on a GSM network should have this). Of course the phone has to be unlocked as well.

Right now KT is the only carrier offering this option. I was in Seoul this summer for a few weeks and was able to buy a prepaid SIM from KT and use it in my T-Mobile US Nexus One phone. I had voice, text, and 3G data for reasonable rates. Full details are at the KT Expat Blog site: http://expatblog.kt.com/. Specifically the "Service Guide -> Prepaid Rate Plans" section. You can also get English-language answers to questions from @olleh_expats on Twitter.

As with most things in Korea, the process is not altogether straightforward and has some quirks:

- You can only get a prepaid SIM card at a KT Global Store (there's a list at the Expat Blog site). It's not available at the airport. Furthermore, you have to wait 2-3 days after arriving in South Korea before you can get the SIM card. This is because the KT store has to verify your passport information against the database of international arrivals, and it takes the Korean immigration agency a couple days to upload new arrivals to the database.

- Your phone has to be approved for use within South Korea. The easiest way to guarantee this is to use a phone that is available for sale in South Korea as well (e.g. Nexus One, Apple iPhone, in addition to *many* other phones). Since it's available for sale in South Korea it has already gotten the necessary approvals. You can check if your phone model is approved by following the instructions on the KT Expat Blog site. If your phone is not already approved, you will have to submit an online request to the appropriate government agency.

- You can add money to your prepaid account at any KT store (it might require some creative pantomime if you don't speak Korean). The stores are everywhere. However, even though the stores are open on weekends, KT's computer system will only allow money to be added to prepaid accounts Monday-Friday. None of the KT staff I spoke with had a good explanation for this.

prepaid SIM cards now available in South Korea
 

It's possible to get a prepaid GSM SIM card in South Korea as of the beginning of this year. Since South Korea never had a 2G GSM network, the SIM card will only work in phones with 3G WCDMA 2100MHz support (most 3G phones on a GSM network should have this). Of course the phone has to be unlocked as well.

Right now KT is the only carrier offering this option. I was in Seoul this summer for a few weeks and was able to buy a prepaid SIM from KT and use it in my T-Mobile US Nexus One phone. I had voice, text, and 3G data for reasonable rates. Full details are at the KT Expat Blog site. Specifically the "Service Guide -> Prepaid Rate Plans" section. You can also get English-language answers to questions from @olleh_expats on Twitter.

As with most things in Korea, the process is not altogether straightforward and has some quirks:

• You can only get a prepaid SIM card at a KT Global Store (there's a list at the Expat Blog site). It's not available at the airport. Furthermore, you have to wait 2-3 days after arriving in South Korea before you can get the SIM card. This is because the KT store has to verify your passport information against the database of international arrivals, and it takes the Korean immigration agency a couple days to upload new arrivals to the database.
• Your phone has to be approved for use within South Korea. The easiest way to guarantee this is to use a phone that is available for sale in South Korea as well (e.g. Nexus One, Apple iPhone, in addition to *many* other phones). Since it's available for sale in South Korea it has already gotten the necessary approvals. You can check if your phone model is approved by following the instructions on the KT Expat Blog site. If your phone is not already approved, you will have to submit an online request to the appropriate government agency.
• You can add money to your prepaid account at any KT store (it might require some creative pantomime if you don't speak Korean). The stores are everywhere. However, even though the stores are open on weekends, KT's computer system will only allow money to be added to prepaid accounts Monday-Friday. None of the KT staff I spoke with had a good explanation for this.

Instead of asking for updates, folks should really just be checking the maintained
GSM coverage map. Japan seems to be the only developed country that is not GSM capable, and S.korea does not have GSM either. That coverage map also shows what data technology is available.

Another good page to keep an eye on is the apn wiki, because it also shows prepaid data plans for many countries.

*edit* I just noticed tylix's post.. then noticed the GSM map apparently erroneously claims to be current as of July 2011.