Most so-called 4G (HSPA+) T-Mobile phones will not work. They use 1700 MHz for 3G/HSPA+ (the UMTS bands on the phone specs). AT&T is buying T-Mobile to gain this 1700 MHz spectrum and change it from UMTS to actual 4G LTE. On the AT&T side they use 850 MHz and 1900 MHz for UMTS. Only the original Vibrant which is no longer sold (not the new 4G update of it) and the upcoming G2x use both 1700 and 1900 UMTS. The G2x goes so far as to also have 850 thrown in as well. Both these phones will get full 3G/HSPA+ speed today on AT&T if unlocked (and probably won't need to unlock after the acquisition completes). No other T-Mobile out today will receive better than 2G EDGE data speeds.
While most of what you say is correct, you left out the T-Mobile branded Motorola Defy, which will work on AT&T's 850 band (at least) for 3G speeds...and on its lower priced "MediaNet" plan. Nice little "ruggedized" phone for $379 (full retail) IMHO.