i have found the very same problem of closed bugs 174012 and 188615, that is when playing a movie with mplayer the monitor turns off after some minutes. the patch (mplayer-dpms.patch) that was written for mplayer-1.0.20070622-r3 ebuild has been dropped in later ebuilds but i think it should be applied back to recent versions of mplayer, too (haven't tested that myself).
updated to mplayer-1.0_rc2_p28288 but the problem persists.
Old bug, please test and reopen if it's a problem with newer ( 1.0_rc2_p20090731 )