When I last left off, MythTV was using my 61MD10 61" HDTV in full 1280x720, but MythVideo was stretching my HDTV .avi files too wide and showing black bars on the top and bottom. As it turns out, mplayer isn't able to autodetect the 16:9 aspect ratio.
Thanks to this post, I found that I needed to supply this information in the MythVideo playback settings.
Here's my new mplayer command:
mplayer -monitoraspect 16:9 -fs -zoom -quiet -vo xv -ao esd %s