This can happen with iMovie 3 or iMovie 4. This does not occur with iMovie HD 5.0.
The solution is to temporarily move the following files from /Library/QuickTime/ to the Desktop:
Final Cut Pro HD
Final Cut Express HD
Restart the computer and try exporting from iMovie to your camera.
When you need to use Final Cut Pro HD or Final Cut Express HD again, drag these files back to /Library/QuickTime/ and restart the computer.