Defeat Apple's Anti-DVD-Screenshot DRM

Apple disables the OS X screenshot capability while a DVD is playing, but there is a workaround.

Using terminal, type in this command.

screencapture -i ~/Desktop/filename.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 "filename.jpg" will appear on your desktop.

Found here with many more options:
http://highschoolblows.blogspot.com/2005/11/take-screenshot-of-dvd-player-in-os-x.html