Just spent some time with my new Rebel T1i and Matt asked how it compared to the 5D I was borrowing, here are some details.
The Rebel is AWESOME!!! Just got a small chance so far to try it out but it's working well so far. I really like the idea of LiveView, it's quite nice actually. I can see it being useful in certain circumstances. The new 900,000 pixel high-res screen may be nicer than my iPod!
The ISO range is much better than the 5D cause of Digic 4 and the improvements in software... the 5D is like 4 or 5 years old and it shows in terms of the ISO and noise-reduction, definitely. Also my camera must be about 1/4 of the weight of that 5D beast. People say they like a more "camera-y" camera, but a "solid body" = lots of weight. The Rebel is much easier to carry around on my neck all day.
I love the IS lens. I took a photo at 1/10 and 4.0 and it looks crisp as lettuce. Movie mode seems awesome so far, though you can't use auto focus cause it's loud as heck and you hear it on the video very clearly. Seems very nice, but manual focus is probably a better choice. 1080p at 20fps is a bit weird, and also HUGE on my monitor, so I will likely stick with 720p at 30fps.
Burst mode rocks, I took like 30 photos at Large/Superfine and it just tore through them, it's got like 3.4fps and that is hot. It's weird that you have to thumb-button/scroll rather than jog-dial to change aperture in manual mode, but I am sure I can get used to that.