Hmmm... You did make me curious now demolition.

This thread talks about HWA, stating it's only possible for the programs that use gstreamer as it's backend, which mplayer does not.

http://talk.maemo.org/showthread.php?t=33127

Googling around brought me to the mplayer site, where it is stated that for nvidia cards (on desktop / laptop machines) there is an option -vdpau.

http://www.mplayerhq.hu/design7/info.html

Unless somebody with more knowledge can point us elsewhere, I'd say HWA is not used by mplayer.