I was messing around with the DVD Player, and I discovered that the new Apple DVD Player support DVD Jackets. More than that, there is a cool Get Disk Info menu item within the File menu. Pick it, and you can rename the Title (and get ride off the ugly uppercase-only text in the title of the window), display region info, and customize the Jacket. You can use any frame you want as a custom Jacket picture.
There's more, though -- the actual frame is saved as a hex stream in a plist in the ~/Library -> Application Support -> DVD Player -> Settings folder. I've read the hex stream using the defaults command, and then copy-pasted the results in a hex editor (like HexEdit). Voila, a perfect JPEG file of my favorite scene. It should be easy to automate the process with little Applescript/perl/python skill.
[robg adds: I haven't tested the image grab aspect of this hint, but the jackets feature is a nice touch; it's basically an image that's displayed when the disc is stopped. If you're serious about DVD screen grabs, though, get a copy of Snapz Pro (or possibly some of the other image capture tools). Not only can you grab a frame, but if you spring for the movie version, you can even capture video clips.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050607220933658