Kai, is your GPU supported (8xxx or higher)?
You also have to use the binary drivers from nvidia (from the rpmfusion nonfree repository)..
If that's the case, please post the outpuf of:
mplayer -vo help|grep vdpau
mplayer -vc help|grep vdpau
PS: -vo is not enough, use the -vc option, too.
Edit: if you don't want to use the -vo vdpau -vc xxx option each time, just edit the config file located in your directory (~/.mplayer/config) and add this:
Mplayer will try to run this configuration first (don't forget the trailing ",").