I have a Mac OS X which has both a built in webcam and microphone, and after using my webacm in other software (Photobooth, iMovie, etc.) it doesn't work on skype. Sometimes it works after I reset my computer and sometimes it doesn't, but it's really annoying to have to resset it over and over.
Generally speaking only one program can access your camera at a time (no sharing). If any of the other programs are set to start up automatically when using your camera, they can interfere with the ability for other programs, like Skype, to directly communicate with it.
The main solution is to either find out what program is hogging the camera and stop it from auto loading (or not releasing the camera) or use a program like ManyCam for Mac that will act as a middle man allowing you to share the camera with multiple programs at a time.
This topic has been covered a few times in the forums and the links below may provide you further insight or a more thorough solution:
In the Flash local settings manager found in "System Preferences", go to the "Camera and Microphone Settings by Site" and add in the following website addresses:
I had the same problem as you - those instructions did not work either. DO NOT move the original plugin that was in the folder into the trash! Keep BOTH the old and the new plugins in that folder and restart Skype again. Best of luck!