Ancient thread, but I was searching on the same problem and it turned this up. In case it helps anybody, I had been running
/usr/bin/xset -display ":0.0" s 900 +dpms
every ten minutes from cron before I'd identified today that mplayer was the problem (and, indeed, there may still be other broken apps that do similar things). Now that I've identified mplayer, I was going to just write a wrapper script that ran mplayer and then restored my settings, or just give vlc a try or something.
The only diagnostics are that when I *do* run mplayer with the -stop-xscreensaver switch (or the un(der)documented -nostop-xscreensaver switch or conf setting), I get
xscreensaver_disable: Could not find XScreenSaver window.
which seems to indicate the switch only applies to the separate xscreensaver program and mplayer unconditionally borks X's own screen blanking/DPMS as a separate thing.
BTW, this happens both on Slackware and Debian (in addition to Gentoo) so it's not distro-specific.
A second BTW is that, of course, for a general use workaround, you'd want to establish what the right display was and what the settings actually were and then restore them and so on.
A proper fix would still be welcome, of course.
-- Edit: oh, and I should say that it's not particularly a KDE problem, since I'm not using that.