Comment 1 for bug 303905

The issue (assuming it is an issue) is that mplayer is running in the background and so its not affected by an X server reset.
A workaround could be to add a signal handler so that on receipt of a SIGHUP gnome-mplayer/gecko-mediaplayer shutdown mplayer.
I tested this workaround successfully for gnome-mplayer with a very crude signal handler (just calling mplayer_shutdown()) which might however not be very safe.