Most of ATT's 3G phones don't have the WCDMA 2100 band necessary to roam on NTT DoCoMo and Softbank's networks. iPhone 3G, Treo and HTC models do have the proper band. The non-smart phones from Nokia, LG and Moto don't.
According to gsmworld.com, the new entrant eMobile has a WCDMA 1700 network. I wonder if that is compatible with the WCDMA 1700 network that T-Mobile will deploy in the states.