well it finally worked.

i made a cube and a nurbs plane -> group it

surface shader -> color out > movie (select your file)

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i used fraps to capture ingame footage
cut a video at its 50th frame, saved as avi (sony vegas)
then used to virtual dub (video->compression->avi(uncompressed rgb/ycbCr))

applied this to the nurbs plane

preferences -> timeslider set to length of my video (50fps)
play every frame

in the the work area double click on the video-node so
that the attribute editor will pop up again

now set a tick in "use image sequence"
you can set the number of images from your video
that should be displayed

animate the cube

rendersettings:

name.#.ext
*.targa

number of frames rendered: were set to 50


i rendered it, when i make a movie out of this
targa files i see a rotating cube and a movie playing on it

cheers