I was not able to get 'grab' to capture the DVD window. I think Apple crippled the software to satisfy the MPAA or something. Or maybe it is just a bug.
I figured out a way to do it though with the standard 10.3 software.
You can use the command line tool "screencapture" in a particular mode. screencapture lets you capture the whole screen, the contents of a window by selecting the window, or a region of the screen by drawing a box with the mouse over the region you want. I found that if a captured a region of the screen that just happened to contain the DVD window then the capture would succeed. If I just tried to capture the DVD window using screencapture's window selection mode it would NOT work. I don't remember if I tried the "whole screen" method screencapture can do.
Here are some instructions from memory/web (not near a mac to verify this works):
1. start Terminal (I think this is in Applications/Utilities/Terminal)
2. type into the terminal window (without quotes, and substitute whatever name you want the output file to be for "desired-filename") "screencapture -i -s ~/desired-filename.pdf"
3. Using the mouse and a click/drag to make the box appear select a region of the screen that includes the DVD window, but that also includes some junk around it.
Once you do this you should have in your home directory a PDF file named desired-filename.pdf (or whatever name you chose) which is the capture of the DVD screen you want plus the extra junk around it. You can edit the image somehow to get rid of the other junk if you want.
That worked for me as far as I remember.
It might be more convenient to capture it to the clipboard though instead of a file. Supposedly this can be done with the following command, althogh I've not tried it and again I'm not near a Mac right now:
screencapture -i -s -c
Hope that works for you.
For more info on screencapture in general see:
http://www.macdevcenter.com/pub/a/mac/2003/02/28/screenshot.html?page=last
