[Yazcedric]

Hi, charlosian.
I always make sighs when I think about the Japanese "closed" cellular market/service.

The availability for Japanese POSTPAID contract depends on your duration of stay in Japan (ie, whether you will enter Japan as a "registered" foreigner).
If you enter Japan for the duration as a traveler (I forgot the maximal duration without obtaining visa), then I would consider Celtrek (you will find detailed report in other thread).
The calling rate is very attractive (compared to Japanese prepaid cellphone).
Also, it looks like Celtrek will provide you a US DID number so that your family members can call you as if they call you as US long distance calls.
The drawback is that people in Japan reach you by making international calls.
Similar issues are also applied when you obtain Japanese cellphone (with Japanese DID number).
So, it all depends how you use (will you call out most of the time, or will you receive calls).

As for postpaid contract, the roaming rate (ie, once you are out of Japan) is terrible, and I don't know how you maintain the account.
For Japanese prepaid cellphones, currently available one (Softbank) is not compatible outside Japan (PDC).

In summary,
1) Celtrek SIM and obtain a compatible unlocked cellphone if you would like to have a cellphone that you will use in the US (or other countries outside Japan).
2) Renting a cellphone may be easiest if you will stay in Japan for a short duration. Also, prepaid cellphone service is not easy (or almost not available) to obtain as a traveler.
3) No Japanese prepaid SIM card package (with Japanese DID number) is currently available.
Softbank offers "renting" SIM card if you bring your own cell phone, but their rate is not so attractive and there is compatibility issue, too.
4) Postpaid contract can be considered depending on your duration of stay in Japan, but these phones are locked to the carriers.
And the roaming rate is terrible once you are outside Japan. I've also heard that Japanese postpaid "SIM" card has some compatibility issues (ie not so "universal" if you try these cards to the phones that are not supported by the carriers).

Hope this will help you.