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.