I would like to add my $0.02. Since apparently I am neither: a) getting the 5d3, or b) switching to Nikon with the D800, I do believe I will just get a new lens.
I have the 17-40L...it's pretty good, especially for a cheap L lens. It has massive distortion, and IMHO only usable from 19mm-20mm upward. It has filter threads, and gives great colors. Sharpness in corners and sides leaves much to be desired.
I would either get the newest Tokina 17-35/F4 or their 16-28/2.8-- which is going for $699 new with rebates on Adorama. Hard to beat that price. It doesn't take filters though, and this obviously sucks for many. In time, there will be a work around.
Also, the Zeiss 18/3.5 or even better, 21/2.8 are supposed to be stellar. Who really cares about manual focus on WAs? Not I