Originally Posted by Gripster
It's not a direct answer to your question, but don't fall for the L-series marketing hype. When shooting with a digital crop body such as the T2i, you may be better served with a good EFS lens.
I have a 70-200 f4L and it's some of the best glass that I own. However, my EFS 17-55 f/2.8 and my EFS 10-22 get great results with my 40D.
This site is a great resource. http://www.the-digital-picture.com/Reviews/
It's not marketing hype, and quite frankly, that's not great advice. If he's looking to spend the money on quality glass, then L or similar lenses are the way to go. EF-S lenses are not a smart purchase for anyone eventually looking to upgrade to full frame. even if you don't upgrade to full frame, L glass is superior to non-L glass in most cases. it comes down to a per-lens basis, ultimately. weather sealing, better optics, better construction...
OP, i'd go with either the 24-105 or 24-70. i prefer the 24-70, and I think that's a lens you'll use for a long time. The 17-40 is not a good lens if you're taking any portraits, and the 24-70 has a full stop of aperture on the 24-105, which could be VERY useful in low light situations.