I bought a Rebel XS last fall as a way to ease in to DSLR-dom.
Things I lust after that the Rebel T2i has (my most recent - "gimme gimme" object):
1. Higher ISO. The XS is limited to 1600, not an issue for most folks, but for a Disney fan it means - Dark Ride pictures are a chore.
2. Faster Focusing. The XS has an older focusing system - the T2i focusing system is the bomb.
Things I look forward to that the Rebel T2i has:
1. Better Sensor. This is really the heart of a DSLR so I'll enjoy the lower
noise, higher resolution, etc. that the newer/better sensor offers.
Things I imagine I'll use but would never miss if they weren't there that the Rebel T2i has:
1. Movie Mode.
2. Better LCD.
Since I already have the XS I wouldn't really consider the XSi or T1i - not enough juice for the squeeze required. After having tried out a 5D MK II (full frame camera) in the store I've also decided it's too much camera for me. So
it's nice to have a focused "gimme gimme' object - I figure about the time I have my money saved up for a T2i I'll have learned enough to know how to use it