More pixels definitely equals better rendition of fine detail, but as we all know, it also means smaller pixel sites and that equates (all other things being equal) to higher noise levels on a smaller chip compared to a large one. See this article for more on that. Sensor size vs. noise is covered fairly near the end.

The tiny number of images I've seen so far from the Canon 5D II, and it must be remembered that these have been on a laptop screen through a mobile BB widget (which reduces image quality by default) does make me wonder if Canon have overstepped the mark by cramming 21 megapixels onto the thing again.