We have been wondering about this ourselves and have seen it posted around several forums and twitter. People have tried unlocking their G2x phone and testing with AT&T SIM cards and everything. Now it looks like our friends at Engadget have got a response from T-Mobile to put this finally to rest. There is no quadband support.
The T-Mobile G2x fact sheet, attached [PDF link], contains accurate information. The T-Mobile website is incorrect and we’re working to correct it. The G2x supports 850/900/1800/1900 MHz for 2G/GPRS only, and supports 3G/4G UMTS/HSPA+ bands I and IV. The G2x does not support AT&T’s 3G bands. This banding is hardware based.
Not only is this bad news, but it sounds like a pretty major #fail on T-Mobile’s part by including wrong information on spec sheets everywhere. It was just too good to be true when we first saw this considering AT&T is in the middle of acquiring T-Mobile and that would be great for a device to work on both. Sadly it looks like that is wrong even though we, and many others reported it back when we first saw the specs of the G2x. For now, enjoy that shiny new phone as Android 2.3 Gingerbread is “Coming Soon”.