Hi, if you've downloaded iMovie 3 and wish to export directly through to iDVD, it won't work if you have iDVD2. Instead, if you export to QuickTime using the Full Quality DV setting, then import that video file into iDVD2, it'll work fine.
Suffice it to say, once i found this out, I ordered iLife!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030210054706233