DVD video is in MPEG-2 format, which is very difficult/impossible to edit beyond basic trimming. You would have to extract the MPEG-2 off the DVD, convert it to DV format, edit that in e.g. iMovie, then compress it back into MPEG-2 when you're all done. Likewise with the Dolby Digital audio on the DVD (assuming that's what it has). You would lose quality because of all the recompressing. It's also not really a job for the faint of heart because it involves multiple steps:
1) Rip and demux (split) the DVD audio/video with a program like MacTheRipper or Handbrake
2) Convert the video to DV with e.g. ffmpegX, and the audio to WAV or something unless you have Final Cut that can handle AC-3 audio
3) Edit
4) Create a DVD with iDVD, or export as DV to burn a DVD with Toast
Unless there is special software I'm not aware of that can edit MPEG-2, iMovie-style...