It really boils down to your shooting style. It is of lesser importance to have super hi res lenses and cameras if you shoot handheld, at low speeds, mostly of moving subjects, as I do. In many, if not most, of the situations I shoot, I encounter these circumstances. I know I could get far superior IQ with a tripod, MLU, lower ISO and all that, but then I wouldn't have the shot. It's not about great IQ, it's about great photographs, be them hi res or not.
Lately I don't feel so comfortable with the high mpx cameras when I shoot with proper tecnique, because of too much resolution. I need more elaborate and careful sharpening routines to obtain the image I like. Sometimes less is more. The sensor size in the 5D and the 1Ds is just right for the regular Canon zooms. Bigger than that and it starts to get fuzzy.
Read Erwin Putts blog The Tao of Leica. He has good stuff on the matter of your question.