1) Put your DVD in your computer and open DVD Player (Applications -> DVD Player) if it does not open automatically. Go to Video -> Maximum Size, or hit Command-3. Fast forward to the frame you want to capture, or select the scene to start at.Link (via Digg)
2) Open the Terminal (Applications -> Utilities -> Terminal). Type this, or copy / paste it right in the Terminal:
screencapture -i ~/Desktop/dvd.jpg
Your mouse should turn into crosshairs. Now hit the space bar. Your mouse should now be a camera. Click the window the DVD is playing in. A file called "dvd.jpg" will appear on your desktop.
Update: Kirk sez, "This is not entirely correct. screencapture's default image format is PNG; adding a .jpg extension does not change this. There's a flag in the man page for screencapture, but the format codes are not documented (and the man page even admits this...). (And I can't get it to work.)
You can change the default format with a simple command - see this article on Mac OS X Hints.