I don't know if you've tried this. But go into your application manager (menu->settings->applications->manage applications) and look for "camera" or whatever it's called I uninstalled mine, so I don't know exactly what it's named. Open that application and scroll down. There should be a "clear default" option. Then back out and try to use the camera. Hopefully it will ask what application to use.
If that doesn't work, there's an application in the market called "ClearDefault." I think it will clear all your defaults (launcher, camera, etc).
Good luck. I hope I helped.