You can probably import them into iMovie (on 06 or earlier, just drag the AVI file to the clips pane and it should import automatically if it's using a codec that QT support; haven't used 08), and then burn the resulting iMovie using iDVD easily enough. I've done this in the past with some web video clips I wanted to mix together.
If you're having trouble with that, you can use a utility like ffmpeg to convert from AVI to a dv stream, which iMovie can definitely import, and iDVD can probably even use directly.