It seems to me that Nikon left the megapixel race before Canon. Nikon D3 has only 12 megapixels, while the big Canons left that behind years ago. And that has allowed Nikon to become Top Dog in the low-noise department and shoot to stardom.
And the new Nikon D90 also has twelve megapixels, whereas the new Canon 50D has fifteen. And according to persistent rumors, the update to the fabulous 12-megapixel Canon 5D will have at least 17 megapixels.
I've decided that until the day I have need for really big specialized prints, if that ever happens, then 12 megapixels is as much as I want. In fact I have prints on my wall in A3 size (about 12x16 inches) froma six-megapixel camera (Nikon D100), and you'll have to do close side-by-side scrutiny to tell the difference between those and some from a 12-megapixel camera. And who does that with wall art? In fact the bigger the print, the greater the viewing distance.
More megapixels than a certain minimum eat up your diskspace and tax your processor for no real gain.
Also, for anything over 12MP, you need really good lenses to tell the difference. We're talking about lenses costing over $1500 and weighing the better part of a kilo.
I hope Canon will wize up and learn from Nikon, and leave room in the line-up for cameras with reasonable MP counts.
I regard this a fortunate last-minute maturing on my part. When I was young I would have thought that if six megapixels was better than three (and it clearly is), then twelve would be better still, and of course twenty-four would be twice as good as twelve.