QuickTime Player.app New Movie Recording Error: won't record
I don't know why this was a problem in the first place, but I was able to resolve the issue from the Activity Monitor so that QuickTime can now access my FaceTime HD Camera:


launch Activity Monitor
select Activity Monitor >> View::All Processes
select the process "VDCAssistant"  (owned by root)
select Activity Monitor >> View::Quit Process
 (also: Option+Cmd+q)


The same can be done from the command line with killall:
sudo killall VDCAssistant

...and here's a more complete listing of things to try if the FaceTime camera has gone wonky.