Hi, Paul!
There are legal reasons to do screen grabs from copyrighted material (academic, for example). I just want to remind everyone that we need to stay away from talking about anything that would violate copyright law.
See the MacOSX.com "Board Rules" at the top of the page for details.
Mac OS X's screen grab apparently will _not_ grab a frame from DVD Player. Assuming legitimate reasons, one probably could use another DVD player such as VLC and do a screen grab from that.
Good luck.
Doug