One more thing, before I get bashed as a sell-out and a megapixel lover, my shooting needs prioritize low-light/high-ISO performance over megapixel count, as the editorial outlets I work for rarely publish images much larger than 16x20. If I'm forced to choose between megapixels or ISO and dynamic range, I'll take the latter every time. However, I'd be lying if I said that I didn't notice a distinct difference between the detail captured by my 5DI vs. my 1DsIII, even when the 5D's files aren't stretched beyond 300 dpi in print. This might have something to do with the low-quality paper that many of my clients use to print their magazines, but it is what it is. I have no control over that and have to make the best of the situation. The 1DsIII's extra 9 megapixels, and the higher dpi that results in print from that higher resolution, is always a welcome plus.
I'm not talking about 200% enlargements, either. At 300 dpi, the 5D's files are 14.5 x 9.7 inches. If the 1DsIII's files are printed at the same 14.5 x 9.7, the extra detail is very noticeable. I suppose that's a long-winded way of saying that as much of a stickler as I am for high-ISO performance and dynamic range, megapixels do matter.
Like lots of people on here, I'm heavily invested in the Canon system, and very reluctant to consider switching over to the dark side. I've been keeping up with all the 5DIII rumors mainly because I think my backup body (5D classic) is in need of an update. Interestingly, it's looking like the D800 might actually end up being a nice upgrade over my 1DsIII, especially since I don't feel inclined to dish out the big bucks for a 1Dx.