Canon and Nikon play leapfrog, trying to outdo each other in the technical and Page Ranking stakes, but at the end of the day they're both very good.
I have a 30D (which of course has now been superseded by the 40D) and I'm happy with it, but I'd probably be happy with a D200 or D300. I've had Canons since 1981 so there has always been an incentive to stick with the brand so I can use existing lenses and accessories. That's the real issue - you're not buying a camera, you're buying a system.
I would say that Canon lenses etc are generally easier to find - Nikon can be in short supply when they release something new - and there's a slightly wider range of third-party accessories. On the other hand, you could argue that Nikon lenses are worth waiting for.
Try a 30D/40D against a D200/D300 in a shop and see which feels best to you. There are quite a few improvements in the latest models, so I would chose between the 40D and the D300 unless money is really tight.
