At least the installer should install a more recent mplayer version by default. I know, it isn't hard to get a current Mplayer package from
http://oss.netfarm.it/mplayer-win32.php and drop the new files over the old mplayer folder in SMPlayer directory, but that should be done automatically, and maybe some kind of list of "stable" Mplayer packages would be useful. I'm currently using r33883, and it seems to be working well, though I didn't really do many tests.