[NoGSMarea]

I have missed the thread for a long time. I hope that I could give some summery from the viewpoint of Japanese.

I have to mention the particular situation of Japanese mobile service. There is (has been) no GSM nor concept like SIM in Japan, at least up to 2G mobile. Just for the recent few years two (Docomo and Softbank) have been introducing 3G (UMTS) service that are compatible with the world wide standard. In their 3G service SIM is used but still now the handsets are provided through the carriers and all of them are SIM-locked.
In Japan we have three carriers, Docomo (practically NTT=the former national telephone company), au by KDDI(=the former monopoly company for international telephone communication) and Softbank.

1. Prepaid service offered by Japanese carriers.

Two Japanese carriers, Softbank and au offer the prepaid service.
They are not 3G-service, so you need handsets that meet their own specification (au>CDMA1x, softbank>PDC).

1)In the case of Softbank, they usually offer the handset and number together. It costs around 5000JPY.
2)In the case of au, they sometimes offer the handset and number together, but not always and more expensive than Softbank. If you have the handset without subscription (number), it is possible to get a new number for prepaid service at au's shops. it cost 4200 JPY. If you can get the handset for au from one of your Japanese friends or from the auction, it is a reasonable option to get a number at au's ahops.

When you try to get a handset, it might be convenient to know Japanese slangs;
KURO(black)-ROM
a handset with the proper subscription (number).
SHIRO(white)-ROM
a handset whose ROM has no information on subscription (number)
HAI(grey)-ROM
a handset whose ROM still has information on subscritop (number) while the subscrition (number) is no more valid.
If you want to get a handset, it should be a SHIRO-ROM. HAI-ROM might be still OK, but there is some risk.

Both carriers require you to show some ID like passport or so, it may varies from shop to shop. It seems that the passport is enough for Softbank. but au may require the ID with Japanese address like Japanese driving permit or foreigner's registration sertificate.
Au's coverage is far better than that of Softbank, I can confirm, because I have both. There is no problem in using CBW services. Incoming calls are free of course.

2. International SIM in JAPAN.
I confirm that both UM and TravelSIM work in Japan. Docomo and Softbank offer 3G UMTS. UM works with Docomo and Softbank but TravelSIM works only with Docomo. Docomo's coverrage is much better than that of Softbank.
But the problem with Docomo is that their network is not completely compatible with the world standard because Docomo has been introducing 3G UMTS from the early time and their old 3G-equipments seem to have compatilibity problem.
In fact USSD and other functions do not work always with Docomo. The TravelSIM's or UM's numbers could not be always reached even if the handset shows that it joins the Docomo's network.
I also mention that Docomo now begins 3G UMTS also in the 850MHz frequency band.

The conclusion would be;
In the case of stay for several months:
According to the law you have to register yourself at the local autonomy's office (city office or town office) in Japan. Therefore you get a foreigners registration ID and can have prepaid mobile service from Japanese carriers.
In the case of short stay;
The easiest solution is to prepare both of UM and TravelSIM before the departure.