Try exporting your movie to the Media Browser then converting to QT or iDVD. That may work. Its been my experience that exporting to the Media Browser will allow the movie to be shared by all iLife programs. If you are still using iLife 08 you may want to get iLife 09. The improvements to all the programs (especially iMovie) are worth the money.
As for the green screen, I have no experience there. Sorry.
1. When trying to export from iMOVIE to .mov format using both the Quicktime export and the export to iDVD the audio no longer syncs with the video. I've tried this several times and for some reason it continues to be a porblem.
2. When trying to add a background to a green screen, iMOVIE does not recognize the backgroud.
Any help on either would be AWESOME!