Regarding AT&T's cutover from their old system (800Mhz analog and/or TDMA) to PCS 1900 (the North American GSM variant that operates in the 1900Mhz band) in the US, I think this is an ongoing project. In the city that my AT&T service is based in (not Houston), they've offered both services for at least a year. They contacted me and tried to get me to switch to PCS 1900, but I opted to keep my old service for variety as my other service is already on a PCS 1900 network (through T-Mobile).

I suspect that Vulcan and Napa are correct in that if you have the PCS 1900 variant of AT&T, the roaming situation in worldwide GSM markets is roughly the same as with T-Mobile from an engineering point of view.

I still suspect that things are not as good if you have Verizon.

Regarding Japan, to my knowledge they currently have no service that's radio-compatible with any other country. Verizon is the only carrier that touts Japan roaming, but they do so with a dedicated Japanese handset that must be rented or purchased separately that they somehow tie in to your home service. I suspect that there are limitations in functionality, such as SMS perhaps not working, and almost certainly call forwarding and voice mail not working properly.

There is one other country that has its own proprietary radio standard: South Korea. To my knowledge, even though they have embracecd CDMA as a base digital standard, only a handset specifically designed for South Korea will work there. One thing that South Korea does have is that their primary carrier has South Korean handsets for rent that accept GSM SIMs. So, if you visit South Korea and have a GSM SIM, whether it be from a worldwide GSM market or a North American PCS 1900 market such as T-Mobile or the GSM AT&T, you can rent a handset that will accept your SIM but whose radio interface is South Korean.