Canon and Nikon dominate the dSLR marketplace. Minolta/Konica just quite the camera business, and SONY is a very distant third in this world.
Both produce excellent cameras and superb lenses. How can one choose between a DigitalRebel XT and a D70 or D50? One factor to consider is which vendor is nastier about keeping their image formats proprietary.
OpenRAW - News is a good place to check. Nikon's D200, for example, encrypts the RAW image format. In general, Nikon has been playing harder ball than Canon in shutting out alternatives to Nikon software. It's not that Canon is being insightful or cooperative, it's rather that Nikon is being more aggressive. OS X support for Nikon is much more limited than support for similar Canon cameras for this reason.
On balance, the Digital Rights Management (DRM) wars favor Canon at this time. On the other hand, if either vendor implemented in-camera DNG they'd win this race.
It won't show up on in any online camera review, but if you're making the Canon vs. Nikon decision, consider Nikon's desire to lock in Nikon customers through the data format. If you choose Canon, consider sending Nikon a nice note.