I’ve noticed a new – positive! – wrinkle since upgrading to iMovie 3.0.2: when you save a project, a QuickTime movie sharing the project’s name appears in the project folder. iMovie automatically generates a DV-formatted QuickTime file of the Timeline contents, almost as if you had exported it manually using the Full Quality DV option. I say “almost” because you’ll notice that the file is much smaller than it should be — it’s actually a reference movie that points to the data files in the Media folder, not a self-contained QuickTime movie.
What’s the point, you ask? You can import that reference movie into another iMovie project (which also imports the source footage, though as a single file, not split between scenes). Or, you could probably use the QuickTime file as the source for importing iMovie 3 material into iDVD 2, although I haven’t had a chance to try it. Either way, it can save you a lengthy export process.