It was a toss-up between the 17-55 f/2.8 and the 70-200 f/4, but in the end I chose the 70-200. Every time I shoot with the 70-200, I love the contrasty colors (without post-processing), the lightning-fast focusing, and the great bokeh. I also shoot wide-open 99% of the time--stopping down gives me little to no extra sharpness since it's already stellar at f/4. I think I'm also more a fan of the distanced, flattened look that shooting with telephotos give you.
The 17-55 isn't quite as sharp wide open and flares much more easily, but for what it's worth, the 17-55 is asked to do much more (I'm under the assumption that designing a USM, wide-to-medium telephoto lens that has both f/2.8 and IS is harder, lens-design-wise, than a simple f/4 telephoto. Correct me if I'm wrong!) It's on my camera maybe 80% of the time and is still an amazing lens, but I had to side with the red ring on this one. 🙂