If you have the video in some other form it is an easier job.
But you can re-create the video from iDVD.
1) You can play the DVD on a DVD player and use an analog-to-digital converter (I use the ADVC300) and run it into iMovie 06.
2) (The way I do it).
Use Cinematize 2, I use it with iMovie 06 (and a few times with iMovie 11) and iDVD 11. It has several modes of converting the data. One mode is called "lossless".