Pretty sure the AT&T 9000 roams in Japan - check with your carrier.

You can rent a SIM card that works on DoCoMo's network while in Japan for about $20/day. Check online. Don't forget that you might need to unlock your phone.

Many SIM rental companies also provide Blackberry service and will send you the SIM before your departure so others will know your number while there.

You can also rent a phone at Narita - there are several stands that provide the service, but then you won't have the number until you arrive.

I go to Japan 6x+ times a year, so I own a pre-paid from Softbank which works great for me, but you need to have a Japan address to buy a pre-paid.