I guess I'm going to be giving an alternate opinion.
I've owned and shot the Canon 20D, 30D, 40D, XTi, and 5D.
I've owned and shot the Nikon D100, D70, D80, and currently shoot with a pair of D300's.
While I've liked my Canons, the current crop of Nikon DSLR's are the best available at the moment. I switched from a pair of 40D's to my current D300's and the improvements were HUGE.
Focus is better, faster, and more accurate on my D300's, and on Nikons in generals.
Nikon is in an entirely different league than Canon when it comes to Quality Control. There is just no comparison. QC is one area where Canon drops the ball badly.
(see my article Have You Had Problems With Your Canon
and Neil Van Neikerk's The Best Camera In The World
where he lists the incredible amount of Canon repairs he's had to do in the last few years.)
In short, I'd totally recommend the Nikon D90. It's a better camera than the Canon 40D or 50D for almost every application.
I'd also consider the D5000 if it fits for how you shoot. Scott Kelby and several others own and love this little beauty.
You can make good photos with any DSLR on the market today, but some will give you better value and be easier to work with. Currently, the Nikons are the best on the market, hands down.
I'm sure Canon will catch up again, but they'd better also bring a whole new level of Quality Control to the game.
See you behind the camera!
Matt Art of the Image