Grabing movies under linux is not a trivial task. This picture is propagated in couple of articles witch are on top of google searchs. The systems are crazy (including some vncserver stuff), and if you want to grab the GL window wich is rendered without X server the things get even more complicated. But..
There is a solutin: xvidcap. It is even packed into .deb file. This solves the problem in an easy way.