Nikon and Canon—eternally locked in battle—do each other good by keeping product quality neck and neck. But in the newest entry-level DSLR shootout, if there has to be a winner, it's Canon's Rebel T1i.
I'm not trying to be all namby pamby, like "Oh, you're each so special in your own ways, it's so hard to choose!" but I can't stress enough how solid both the Nikon D5000 and the Canon Rebel T1i are. For the price—$850 for the D5000, $900 for the T1i, both including beginner-level 18-55mm lenses—either one will work fine, whether you're a beginner looking to learn about photography, or a veteran with investment in a few lenses but not enough cash for a step-up Nikon D90 or D700, or a Canon 5D Mark II. (I highlighted their spec differences here.)