[Yazcedric]

Hi, Malkav,

I don't think you can purchase a new prepaid package by showing just your UK passport (unless the folks at the cellphone shop simply forget/ignore their company's policy).
Please see my post in the other thread regarding the "official" regulations requested by the Japanese carriers; see "phone help" thread in this Africa/Asia/Pacific category.
I carefully read their "official" requirement to apply a new prepaid/postpaid cellphone package in Docomo and Softbank's website.
Now I also read AU's website (Au operates CDMA prepaid cell network).

Here is my understanding.
To apply a new prepaid cell phone,
you need to show your registration as an alien (and its validity has to be longer than 90 days).
This regulation is applied to both Au and Softbank's applications for a new prepaid customer.
This alien registration is issued only after you enter Japan (by the local municipal government office where you will "stay" most of your time in Japan) and the validity is dictated by your visa status (ie, unless you have a visa that allows you to stay in Japan longer than 90 days, the validity is restricted).
Per Japanese government's website, they say it is NOT required to obtain alien registration if the duration of your stay is less than 90 days.

As a postpaid contract, Docomo seems to offer a new contract for non-Japanese with an alien registration (they specifically mention that you need to pay by your credit card if you have a shorter validity than 90 days).
I think it is probably due to the fact that you cannot open a bank account with a short validity of alien registration.

In short, I don't think it is worth going to these routes unless you will stay in Japan longer than 90 days.

Therefore, as a traveler (stay less than 90 days), I would go for
a) international roaming SIM card with compatible unlocked cellphone (I think DRNewcomb has good review about his recent trip to Japan using Celtrek card; TravelSIM or UM may be another choice)
b) rental cellular phone (or rental SIM card) from Softbank (I saw their advertisement in their website)

Calling rates are actually more attractive with Celtrek than Japanese rental/prepaid package. However, DID will be US number (not so convenient for the local folks who will call you in Japan and visitors from non-US countries).
Latter is useful if you receive most of the calls from local folks since DID will be Japanese cellular number.
I think latter option is more reliable (connectivity, etc) since it is offered by the Japanese carrier (ie not roaming like the international SIMs).

Of course, if you already have either one of these options, you might try purchasing a prepaid package (and see how your attempt may work!).
Please remember, both Au and Softbank's prepaid phones are useless once you leave Japan.
I keep my Vodafone Japan's prepaid since I visit Japan at least once a year.

I am sure that there are some markets to sell prepaid cellphones anonymously (prepaid cellphone already activated under somebody's name=not your name), but I would stay away from it.
These sellers DO know that customers need to show their ID or alien registration to purchase a prepaid phone package AND they will SELL it.
Prior to 2004, there was essentially NO requirement for showing IDs to apply prepaid cellphones in Japan.
Unfortunately, due to its anonymity, there were bozos who "abused" these cellphones for their criminal activity.
As a consequence, it is very difficult to purchase a new prepaid phone as a traveler.
It's just like everything else (some small number of people with bad intention will ruin the whole benefit of service)...