I’ve discovered since I’ve started using rotated monitors with mplayer that any media files play back at a weird aspect—stretched really tall. I figure it had something to do with rotated screens (I run at 1600×1200 resolution, but rotated 90° clockwise). I finally found the correction:
Edit your ~/.mplayer/config file and add the following line:
monitoraspect=3:4
Since normal-aspect monitors use the 4:3 ratio, but I use mine rotated, I have to invert the numbers. This fixes the problem.