In iMovie HD ('06), you have an option for Share > QuickTime > Full Quality. You don't have that option in iMovie '09, but you can still do it pretty easily. You would use these settings if you wanted a copy of the original movie that is just as good as the original footage (little compression). This would not be appropriate for posting to the web. Here's how to get a Full Quality Export in iMovie '09 (and '08).
