James - I would second the Sigma 10-20 for a Nikon mount. It'll give you a whole new perspective, the image quality is very good, and I think it'll be your least expensive UWA (ultra wide angle) option for a Nikon mount.
Andrew - If you are talking about going with a UWA, you could go with a Tokina 11-16 which will give a constant 2.8, and it's built like a tank, but you are limited in the focal length. You can also go with the Sigma 10-20 (canon mount) which is a fine lens, although 3.5 is the quickest it'll be, but frankly, if you have the extra $'s, I'd go with a Canon 10-22. Even though it "only" goes to 3.5, the image quality will be better than the Tokina. I'm renting the Canon 10-22 for 2 weeks in June to stick on the front of my XSi.