use dpkg NOT apt-get, apt-get removes programs which depend on mplayer too which makes up for a lot of time reinstalling the same programs.

Anyway:
sudo gainroot
killall apt-worker
dpkg -r --force-all mplayer

Then grab your version from http://repository.maemo.org/extras-d...ree/m/mplayer/ and dump it on a memory card and then
dpkg -i /media/mmc2/mplayer_1.0rc1-maemo.<press tab>