Have you had the same problem and frustration I have had for months with my iSight cam not working in Skype??? Well I have finally found a solution which works for me and hope it will assist others with this problem.
I found that I needed to remove a QuickTime plugin called CamCamX5.component and to do this just follow these instructions ...
First of all, make sure you have closed Skype and iChat (Messages as it is now called in Mountain Lion). Then to locate the plugin file (CamCamX5.component), either do a search of your hard drive to find it or go straight to the folder where it is located i.e. Macintosh HD\Library\QuickTime where "Macintosh HD" is the name of my hard drive.
Once located you can either delete the file competely or move the file out of the Quicktime folder to another location. Either way you will need to authorise the moving/deleting of the file by entering your administrator password.
Now restart Skype, open Preferences and select the Audio/Video button to display the Audio/Video options. The camera "Built-in iSight should be selected and your iSight camera should now be working.
Hope this works for you as it did for me !!
Oh and by the way, as a bonus my Skype has also stopped hanging at start up which seems to me to be related to my iSight cam not working in Skype.