Sounds like a good deal on the Canon lens. I know the Nikon 12-24 f4.0 is nearly $1000, although highly rated, and the Canon 10-22 is probably in the $700 range normally. I'm considering the same class of lens, to replace my old Canon 20-35 f3.5/5.6 (from the film days with my old EOS A2 and trusty EOS 630.) But, I got a great deal on a Nikon D50 kit ($489) so I'm giving the Nikon side a tryout.
I've read some interesting reviews on available lenses from several different sites (here's
one link,) and the Tokina 12-24 looks like a possible standout (pro build quality) both optically, and at $499 it's half the price of the Nikon. On the Canon mount it has built-in focus motor like the Canon AF lenses, but the Nikon version still uses the older Nikon autofocus system using a cam-type focus connection to the camera body. Most of the newer Nikon lenses are like the Canons with ultrasonic type.
Still sorting out the Nikon ergonomics after so many years shooting EOS. If I get comfortable with the system, I might start adding some lenses, otherwise I'll eventually spring for my dream camera, the EOS 30D.
I'll keep you posted on how this Nikon experiment works out. Some quirks already turning up...