I'm the first to speak of the wonderful things that Canon do, but that said, I'd love a D3
The reason that I shoot Canon is that when I bought my 10D nearly 7 years ago it was the best on offer in my price range. Now that I have over $15,000 invested in various bits of Canon glass and other accessories, I'm staying with Canon.
The switch to Nikon (or any other "emerging" brand) would simply be too big a hit on the bank balance.