A few additional differences:
1) Backlight on the 7230 is very weak relative to the 7290 (and also the 7100)
2) The 7230 only has 16MB memory, which really is a tight fit with the 4.0 operating system.
3) The T-Mobile 7290 is a lighter blue than the 7230. Cingular 7290s are black.
4) Many/all newer 7290s are locked to Cingular and cannot be unlocked easily.