I wrote too soon; the problem's still there. The only way I can get the tablet to dim immediately (or after one second) after a long session on mplayer (at least the length of a Futurama feature movie), is by going into Control Panel, change the settings of screen to something else, lock screen and keys and wait for the new setting to run its course (luckily there's a 30 second setting now), unlock it, go back into Control Panel and put the settings back to 5 minutes each. Only then will my tablet dim the way it should.

Mplayer does not appear to hang; at least, neither ps nor top show it running after exiting.

Disabling the screen dimming (while mplayer is running) is done by calling a function which blocks the screen timeout for 1 minute; this is called something like (iirc) every 30 seconds, so it would be a minimum of 30 to 60 seconds after mplayer exits till screen dimming would be expected to act normally. Are your troubles occurring in this time window, or later?

They are persistent until I perform the procedure denoted above.