If I may step in: I have been using the 5d for 1 and a half year now, it was the first affordable FF DSRL, even the first affordable high rez DSLR at that time, I use it with a TS-E lens and a few other lenses.
I am a bit surprised that some give the 5d so good notes, I agree detail/ resolution and ISO noise are quite good, although I don't like that color noise in the shadows that can show up quickly at higher ISOs, what bothers me most is the limited DR and the colors. Before the 5d I had Fuji S3 and while it's camera body left a lot to wish, the colors of the FUjis was much more "photographic" that the Canon color, subtle tonality better, and color palette more film like. Canon with C1 and specific profiles (Magne Nielsen) looks precise but always a bit "technical" in comparision, not really beautiful. Maybe people with better processing skills may pull out better results than I do.
I expect the most important advances of the next 5d or 6d in the highlight priority mode that protects the highlights by 1 stop of burning out, and maybe 14bit color. I often find the DR of the 5d limiting.
I have seen about 10 full rez samples of the 1ds3, they all looked quite bad, no detail on a pixel level worth a second look. Michael Reichmann has a first look" on it but apparently (had to) pull back a paragraph where he critically discusses the lack of extra resolution over the 1ds2 and the negative influence of the apparently too strong AA filter.
All in all it seems that at a given format - like 35mm one can only get a certain level of image quality, above the progresses get very small. Maybe that sweet spot of resolution is around 14-16MP for FF 35mm format, best lenses required (find an excellent WA lens for Canon uughh).
I personally want to get another camera next to my 5d and I am torn between the M8 (for weight/ sharpness/ lens quality) and a second hand ZD SLR (not back) that can be found for good prices now. The Leica has the advantage of portability, size and the terrific lenses for that system (also Voigtlaender), the ZD has a very narrow "sweet spot" of useability, only ISO50-100 really, so you have to have your (heavy) tripod with you all the time, but I guess when shot right the pics can easily outperform anything a Canon DSLR can do. Personally the 1ds3 does not attract me much, specially at the premium price of 8k.
But all that is of course my subjective impression...
regards, bernie