I've got nothing against any other brands (pentax, leica, nikon, etc.) and I'd switch in a heartbeat if necessary. The D800 looks like a great camera, but with that said, I'd rather prefer the 5DIII and I want one badly. For me the high FPS makes me actually prefer it. The D800 and larger file resolution does not make me want to switch at all. From what I can see, enlarging a 5DIII file or reducing a D800 file results in approximately the same end product, and I have enough editing skills to make either end file do what it needs to for a final print. If I really need the higher resolution, I'll gladly wait for Canon's upcoming high mega-pixel release or if I could afford it (which I can't) just go to medium format.
Having a 7D, 5DIII and Canon's upcoming high-mp release actually sounds quite good but if the high-mp release doesn't far exceed the D800 while keeping the features of the 5DIII I would without question simply get 2x 5DIII's instead. I could buy a 5DIII now (I'm not holding out for any other camera release or system switch) but I think I'm going to continue to invest in Canon glass at the moment and pick one up when the first or second wave of sales rolls in, the price is a bit hefty but understandable. I really want one right now though and if it weren't for reports of a potential need for a recall or update and the stock shortages I might consider just jumping the gun and putting it on my photography/computer credit line, the machine looks amazing and is basically everything I wanted out of a non-1D/permanent battery gripped sized body. I would not be surprised if I got one before the end of the year or sooner, or just shortly there after. Just ordered a 135 f/2L about 40 minutes ago and looking for more glass.