I have both cameras (upgraded to T1i, but still have and occasionally use Xti). The biggest improvements are the larger, sharper LCD, Live View, ALO and HTP. The additonal pixels are good, I suppose, but I've never made prints bigger than 11x14 anyway. After having the camera for more than a year, I've never even tried (this is not a joke) the movie feature on the T1i.
In my mind, the upgrade is worth it for the LCD alone.
60D, EF-S 10-22mm f/3.5-4.5, EF-S 18-135mm 3.5-5.6 IS, EF-S 55-250mm f/4-5.6 IS, Tamron 17-50mm f/2.8, EF 28mm f/1.8, EF 50mm f/1.8, EF-S 60mm f/2.8 macro, EF 85mm f/1.8, 430ex, 220ex, Alien Bee B400 (2), Alien Bee B800 (2)