More specifically, the AT&T phones that should work in Japan besides the iPhone 3G are the Palm Treo 750, and AT&T Tilt (rebadged HTC TyTN II).

T-Mobile has only carried a few different 3G phones and they are likely to be only UMTS AWS (2100MHz downlink, 1700MHz uplink), which is not deployed in Japan. However, the new T-Mobile G1 Google Phone is supposed to support both UMTS AWS and UMTS 2100MHz and should work in Japan. I have one on order for delivery on Oct. 22.

From what I've read, EMOBILE is using UMTS 1700MHz (both for up and down links) and is not compatible with T-Mobile US 3G phones.