Hey guys! I know I'm replying to an old thread here, but I was having this exact issue today and wanted to post my solution to the internet somewhere.
The iSight camera has no settings, and adjusts it's framerate by the amount of light available during the recording. With adequate light it records at 30 fps, but will drop below that in low light. If the frame rate is something lower than the project settings, there will be gradual audio sync loss in the project. If you're recording a long clip in iMovie and want the audio to be in sync throughout, you need to be in bright light. I'd suggest being near a window and using daylight.
I know this doesn't help those who have already recorded material, but this method can be used to prevent future synch issues when using import from camera with iSight in iMovie.
