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i have the same problem.

You could solve this problem with installing the nvidia
binary drivers, and set up clone displaying, and have your
monitor on 1024x704 (or whatever youd like), and tv on
800x600 or something..

or you could change your monitor resolution to match with
your tv res.
i simply do this because i have problems with the nvidia
drivers (x locks up on my tnt2 card), and whenever i want to
watch a movie, i change my monitor res with xrandr -s
800x600 (i think this is utility thats coming with xorg),
then i start nvtv and pick out 800x600 and click tv-on.
Now i can use mplayer fullscreen perfectly :)