Using Skype for the Mac, the video was blank and could not do a video chat. Turned out that iChat was locking the camera. To unlock it, open iChat, go to Preferences, in the Audio/Video tab, unselect “Open iChat when external camera is turned on”. Close iChat and restart Skype. Now, in Skype preferences, under Audio/Video, video should be working.
More info:
Isight not working