Every time I inch closer to buying that Sigma 30mm, I find something else I can do with my standard issue $100 Nikon 50mm f/1.8. It's just amazing how much quality you can get out of the cheapest lens in the lineup. I assume Canon's 50mm primes are similarly good value for the money (and fully compatible with full-frame cameras).

At least on the Nikon side, I have no fear of my cropped-sensor lenses being obsoleted any time soon. Nikon has steadfastly stuck with their DX sensor, for good or for bad. If Nikon's pro cameras eventually move to full frame or something, then I'd be thrilled to pick up a used D2X. With Canon, I suppose, you need to assume a mindset of "get what I need for what I've got". If you've got a 1.6x body and you need a wide-angle lens, then get the cropped lens, use it, and worry about ditching it if and only if you eventually get a different body. Otherwise, don't worry and be happy. (Or, buy Nikon and just be happy )