Do you have another video device plugged in? iMovieHD has the option to switch input devices when you switch the capture device (right click on the camera). Look at the toggle that switches between capture/playback/editing. I found this out one day when our DVcamera was plugged into firewire and the iSight was showing as the input for capture in iMovie.
It doesn't sound like it will fix your problem, but its worth a look.