I've been debating the switch to Canon's 1Ds Mark II but opted to give Nikon another few years. In part it was an economic decision but also because the Mark II is a bit too heavy and actually makes files too big for my day to day needs. 12.4 megapixels is just fine. Plus it's not just about the megapixels. I wanted the familiarity that comes with Nikon and the weight issue is a real concern for me. After holding a camera for twelve hours straight, every ounce matters and Canon's camera is really heavy. That said, for anyone debating between Nikon and Canon (and you don't have any lenses yet), I'd urge you to buy Canon. I can only hope that the next few years sees Nikon pick up the ball they've clearly dropped and I won't ever have to enter that debate again.
Take pictures and save your money.