I was recently in the same position as you and chose the 17-55 between the 3 you listed. I've only had it a few weeks but it is an awesome lens and have no regrets. I posted some pics in the thread I started.
http://archive.reefcentral.com/forum...readid=1278575 The colors and clarity are amazing when compared to the 18-55 kit lens.
I also had worries about the dust issues but everyone that has the problem also say that it doesnt effect image quality.
I went with this lens because of the IS and the fact that its a little smaller than the 24-70. The plastic body of the XTi doesnt balance well with the heavier lenses and I was looking for a lens that stays on the camera most of the time so something more balanced would be more comfortable hanging off your neck.