When I first plugged my Canon Optura 200MC into my new 24" iMac, this is what I got:
What that is showing is my movie, but translucent, with the desktop underneath. Note how you can see my web browser through my iMovie window.
I've never seen this before, have you?
Turning off and on my camera changed nothing. Restarting iMovie changed nothing. Replugging in my Firewire cable did nothing.
Finally I logged out and logged back and it got fixed.