It's a (relatively) simple matter of using a radio and antenna that supports 1700/2100 and 1900/2100.
ATT and TMO are both using HSPA, it's not like GSM and CDMA. The only major issue there is the band, and they're pretty close together as well, so antennas shouldn't be a problem.
This is only a temporary problem, the AWS 1700/2100 band is still young. Eventually all major handset radios will support it.
