Rather than adding to this thread about SIM cards, I'm starting a new thread now because the scope of our communication options/needs has changed a great deal during the course of the past 2 years.

While casual tourists should be able to get by with a simple SIM card purchased from a newspaper stand, anyone who makes multiple trips per year and/or plans on spending a lot of time doing ground transit should book 2 hours early during their trip in order to go to an electronics market; I'm not talking about Dazhong Dianqi or its peers; you need to get local!

This is fresh in my mind because I did the entire drill just yesterday for a guest who requires 24/7 access to Bloomberg. Here are my tips:

1) when you enter the market, you will encounter many people who want to help you buy
*I used to think "screw that"; I can get better prices if I shop for each item individually
*but, I've come to realize that having one person take care of all of my needs saves me a considerable amount of time, plus I get to hold him accountable if anything breaks... well worth whatever kickbacks he receives
*just try to pick a person who looks friendly, probably not on the first floor of any given market

2) the wireless router
*single best purchase I've made during 2012, to date
*Unicom WCDMA is the fastest, and also the most expensive in terms of data pricing; having used (very slow) China Telecom in the past, I would not think twice about paying the Unicom premium (we're not talking about big bucks here)
*the Hame router, for which I paid y600, now costs y450, and the Huawei router is y700; my guest went for the latter because the merchant told us it was a better brand

2a) WCDMA SIM cards
*as far as I can tell, they don't have phone numbers attached to them, and your iPhones don't support them anyway, so if you buy one, think about data only
*data costs y80/gig
*the y300 cards sell at face value, and last for 6 months
*the y1200 cards sell for y900, and last for one year
*there is a minimum monthly usage charge

3) SIM card for your phone
*if you have a smart phone, Unicom is probably the way to go because very few non Chinese phones support China Mobile 3g
*3g Unicom SIMs cost y150, including y50 in credit, though I've heard you can knock the price down to y100

4) spare phone
*if you have an iPhone or similar, you should pay y50 to jailbreak it so you can use it with your new Unicom SIM
*but, if you don't want to miss calls to your Verizon number, you should consider picking up a spare phone; a pretty decent base level Nokia will set you back no more than y250 (y150 if you want go really, really ghetto)

5) universal USB charger
*these cost a mere y20, and can charge pretty much any mobile device under the sun via your computer's USB port

6) travel router
*many hotels in China don't offer in-room wifi, but you can solve this problem by purchasing a travel router (plug it into the wall socket, and connect it to the ethernet port)
*cost = y120 (sure, Fry's and Amazon might be cheaper, but since you're already in the electronics market, why not pick one up?)

7) headset with microphone (self-explanatory; they are cheap, and will permit you to use Skype/gchat with less "are you there?" moments)

8) configuration of devices
*before you leave the market, implore your "guy" to make sure that everything works, sets your SSID/password to terms of your choosing, and changes the software interface to English (not always possible)
*be sure to get his business card

9) haggling
*I encourage you to try your best on this front, but don't expect any miracles
*20% off on certain products is doable; for low margin items, you may well be stuck with their listed prices

10) market locations
-per jiejie's suggestion, I'm adding this section here, which I will try to keep updated based on user suggestions
-Beijing
*I can't recall the name of my favorite market, but it is half a block north and half a block west of Chef Too; I will try to update this later
*Buy Now: This is a little more expensive than the aforementioned location, and traffic in the area can be intense; I prefer the newer complex on the northwest corner over its sister across the intersection
*Zhongguancun: Generally cheaper than both of the above, but more time consuming because you need to make the trek up there, and it is extremely massive
-Shanghai
*Xiangyang Electronics Market
-Shenzhen
*Hua Qiang Bei: sort of like Zhongguancun on steroids
*Since Shenzhen it takes an hour+ to cross town in Shenzhen, and electronics are the lifeblood of the city, I recommend finding a smaller market close to where you happen to be; all districts have them