I can't believe it! What on earth is Canon doing! They once had the lead over Nikon, not only with regard to their offering of a full-frame format, but also with regard to significantly lower noise at high ISO in their cropped formats, such as the 20D and 30D.
Nikon changed that just a very few years ago with the introduction of the D3 full-frame format which in terms of dynamic range and low noise, exceeded anything that Canon had to offer.
Nikon has subsequently continued from strength to strength, whipping Canon with every new release. The D7000 was a breakthrough in DR capability, and now the D800 is a breakthrough in both performance and pixel count, if one assumes the pixels are at least the equivalent of the D7000 pixels.
I've got two 12mp full-frame cameras, the original 5D and the Nikon D700 which has better specs than the 5D. Since I have quite a few Canon lenses, including the highly acclaimed TS-E 17mm, I would be very interested in a 45mp or even a 36mp 5D MkIII.
I doubt very much that I'll buy this new offering from Canon, unless it includes some other breakthrough in performance which hasn't been announced, such as spectacularly low noise at high ISO along the lines of the Nikon D3s.