It looks like many people are complaining about a disappearing onboard iSight when upgrading to Snow Leopard, whether it's a fresh install or not. I personally experienced that under Tiger and Leopard. It took me days to figure out where it came from, but I suspected that some software I eventually added was responsible for the problem.
The last time I got the iSight working again, I can't even remember how it happened. Since I'm not very careful when playing with fragile things on Mac OS X, it happens that I break something then get it fixed but don't know how.
Here is a link that saved a bunch of time in troubleshooting such problems, though: Kext Utility x64 v2.3.2
This app simply fixes permissions -- specifically on .kext folder -- so that kernel extensions can be used properly. Moreover, it accepts a .kext file as an argument when drag/dropping a .kext icon on its own icon. So I simply dropped an AppleUSBVideoSupport.kext file icon on it, and that fixed my camera.
I was quite sure this .kext was present, but the program said it wasn't. Don't ask how/why. I'm not sure this hint is very valuable, since I can't add any added value of my own, but the app did work.
[robg adds: I haven't tested this app, and it seems (from reading the associated comment threads) that it's really for those who muck about with system extensions fairly regularly. Still, if you're having an issue with an onboard piece of hardware not working, it might be worth a shot as a fixit tool.]
Mac OS X Hints