I had some 'fun' this weekend while trying to edit the video from my nieces' christening. Basically if you create a project in iMovie and save it to a volume that's FAT32 (Windows/DOS) or UNIX rather than Mac OS Extended, you'lll have issues. You'll find that after you save all the changes to the project then open it again, all the edits you made are lost. The imported clips are in the media directory (and it asks if you want to delete the stray clips) for the project but the project file itself isn't saved.
I didn't even think of re-formatting my external drive when I bought it, and the result was all of Monday I had prepared this movie (and was quite pleased with it), only to find the edits not saved. I just though I'd save someone else the same hassle.
[robg adds: I can't test this one. Can anyone confirm?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040505061229615