Thought I'd gather some info on prepaid SIM cards in Japan seeing as I would also like to ask you the same ^^
The most popular option seems to be Softbank where you rent a SIM card for 105 yen per day and then pay 105 yen per minute. Free incoming calls. You need to return the SIM though which is inconvenient. They do have data plans but you have to use the 703SHf phone which they rent out for 250 yen per day. The data then costs 0.32 yen per packet - have no idea what that means ^^; The phone has a PC browser so that you can check your GMail or whatever. Unlikely that you can tether the phone to your laptop though.
Softbank do have a prepaid service for folks who are staying in Japan for a bit longer and are available to those who have more than 90 days on their visa > http://mb.softbank.jp/en/prepaid_service/
AU have pre paid plans which they list up here but you have to use them in an AU phone... Hello?
Docomo is the national carrier belonging to NTT. They recently announced that they are getting out of the prepaid market and will end their current prepaid services in 2012. They sold their last prepaid phones in 2005. No prepaid SIMS available either.
So, if you need to be able to keep in touch with folks via the phone then go for Softbank. If you need data too and prefer to use your own mobile device then sign up for a HotSpot account as they seem to have quite a few spots around Tokyo including on the subway.
If you don't mind what you check your mail on then get the Softbank 703SHf mobile mentioned above.
If you are staying long term in Japan then I would personally recommend Softbank who are the official carrier for the iPhone. Most of my comrades have an iPhone which means that we can call each other for free between 1AM and 9PM! The data plan is reasonable too and costs 4410 yen per month for unlimited usage.
I started off with Docomo and then switched to AU as I wanted a more functional device. Then jumped ship on AU for Softbank as I wanted an iPhone.
I'll be traveling to Hong Kong, France and the UK soon and was hoping that maybe some of you could advise me on what prepaid SIMs I should go for - me need voice and unlimited data. Will be using my accidentally jailbroken phone. And while we are at it, I thought it would be useful for folks around the world to chip in with info regarding prepaid SIMs in your neck of the woods too.
Found this for the UK > http://threestore.three.co.uk/sim/default.aspx Unlimited Internets for 15 pounds a month? @.@
For Hong Kong I found this for 78$ which is supposedly unlimited data > http://www.hk.chinamobile.com/p_prepaid_mobiledatasim_en.jsp
Couldn't find anything for France and just read about how somebody was paying 30 euros per megabyte @.@
Wondering about folks who have visited Japan. What did you do for prepaid solutions? Did you just use your roaming service with your mobile?