There a bunch of ways to get movie files off a DVD (mostly covered earlier), and they all use conversion unless you can use the VOB files directly. VOB files can be used directly if they aren't encrypted at the DVD replication factory; sometimes you can just import them into an app like Adobe Premiere (rename them to .MPG first) or Avid Liquid. On the Mac, you'd need Final Cut or Apple's QuickTime MPEG-2 Playback Component.
You can also convert the DVD file into some more common format like QuickTime using free apps like these:
MPEG Streamclip (Mac/Win)
Premiere Elements trial (Win)
Update: Some people prefer professional apps like Cinematize (Mac/Win) and TMPGEnc 4.0 XPress.