I think Mac OS 9 will allow you to have files over 2GB, but I don't think iMovie will let you. It always breaks my files into 2GB segments as I'm importing (if they weren't from a DV camera that had scene breaks in it already). They play seamlessly, though, and I would really hate to have one 12GB file go bad on me...
...not that that would EVER happen on my Cube....
-Doug