Basically, it is a 3G band issue. TMobile & AT&T Wireless do not use the same 3G bands (AT&T Wireless does not use use the conventional ones). G2/HTC-Magic was built for TMobile.
The G2/HTC-Magic 3G Network HSDPA 900 / 2100
AT&T Wireless uses 850mhz(the old TDMA band) and 1900mhz for 3G.
Ho, well, it was too good to be true. Edge is still not that bad, at least for email/calendar and even Map.