I don't make many movies and I use a card reader not direct camera hook up but I have seen what you say. How I get around it is I make sure iMovie is open, I dismount the card and then remount it and iMovie find the files on it. It seems if iMovie does doesn't see your card or I suspect camera it reverts to the camera on the computer. There may be other way as well.
I was having the same issue. Could not directly import video from GF1 to iMovie 11. Had to use Handbrake first. Apple introduced an upgrade to iMovie 11 yesterday, and it added some camera compatibility. Problem solved!
iMovie 11 need reads video file created using the GF1.