I am looking at getting a lens that can do 2.8.
I have a 650D/T4i. I have been looking at the 40mm pancake, but its fixed range and no IS are giving me sour grapes. I shoot mostly handheld. I know it has gotten great reviews and for the price you can't really beat it.
However, the EF-S 17-55mm f/2.8 IS USM has got some pretty good reviews, and has the advantage of more range and IS. The price point is in a totally different league, of course.
If you didn't have either lenses and had the budget for the EF-S 17-55mm f/2.8 IS USM wouldn't it make more sense to get it over a fixed 40mm that can do the same 2.8.
EDIT: Ooops, I should have put this in the Canon EF and EF-S Lenses section. Sorry!