This is a question I have, too. I figured this out on Windows: how to extract a specific segment from a DVD and convert for use elsewhere (PowerPoint, in my case). I've not accomplished this on the Mac, largely because I don't want to pay $50 for QT Pro and the MPEG2 Playback codec. But, I think this process would work (but I've not tested it, so I can't guarantee this is $50 well spent).
Buy
QuickTime Pro and
Mpeg2 Playback component
Rip DVD
Open VOB file(s) with clip of interest into QuickTime Pro
Use QT Pro to trim clip and export as H.264
Import H.264 into iMovie 08
Edit as desired