Mind you if you have an nVidia graphics card you should not only specify the -vo device but also give a -vc setting. On my own system I give the following on the commandline:

mplayer -vo vdpau filename

(replacing 'filename' with the actual name of your file of course) and place the following in ~/.mplayer/config:

[vo.vdpau]

vc=ffmpeg12vdpau,ffwmv3vdpau,ffvc1vdpau,ffh264vdpau,ffodivxvdpau,

but there are several different ways of doing this. This method allows you to use different video out devices with MPlayer but will give proper -vc values when you specify -vo vdpau from the commandline.

Hope this helps :).