At one point, it was possible to screencap from a Mac with an Nvidia video card, but not an ATi card. I remember talk from many months ago that Apple was going to close the Nvidia loophole, but I didn't hear about it again.
I have a PowerBook G4 with the ATi Mobile Radeon 9600. Screencaps cannot be done on this system using Apple DVD Player. I've tried DVD Capture and that doesn't work. The only way to screencap on my machine is to use VLC, but since VLC doesn't have a "step-frame" function like Apple DVD Player, it can be a torturous process to get the picture you want.
I would be interested to heard from those who have successfully screencapped on their system: what video card they have, what software they used, and what OS version they have.
Allow me a moment to finish eating my crow. (Hmmm. Good bird.)
I doubted that any method would work because I have tried screencap programs in the past, including DVD Capture. They never worked. And all my talk about Nvidia and ATi stands; there is a little bit of discussion about this issue at the DVD Capture feedback at MacUpdate.
However, I decided to try ScreenCaptureGUI, and it worked like a charm! (Really ugly interface, though.) It must get the source for the video feed at a lower level than DVD Capture.