It’s been on Digg and Boing Boing, but here’s a guide to taking screenshots while playing a DVD using DVD Player in Mac OS X. It involves dropping to the command line and using he screencapture command.
There is, however, an easier way – just download VLC. By playing your DVDs in this you can use Grab as normal, or cmd+shift+3/4. In fact, VLC has a menu item on the Video menu called Snapshot, however this wouldn’t work for me.
This is just one more reason why I love VLC – it’s now my default media player on both Windows and OS X. Its DVD playback is especially good as you can skip the opening sequences and go straight to the menu – handy on discs like Shrek 2 which have around 10 minutes of unskippable trailers. It can also often playback discs from any region, even if your drive is set to a specific one, though I think Apple now sets the region in its hardware so this probably won’t work on newer Macs.