I was having the same problem in Mepis 6.0-1, and found this thread while trying to solve the problem. None of the fixes listed above worked for me, but I did find a (rather strange) fix that worked for me so I thought I'd add it here. If I turn on the screensaver and have it set to go off either before or concurrent with standby (in pwr mgmt section) and then set suspend to go off after standby, the screensaver kicks in when suspend
should. I haven't tried all combinations, but the order of the triggers does seem to make a difference.
For example, if I set the screensaver time to 1 min, standby to 1 min, and suspend to 2 min the screen goes black (backlight still on) after 1 min, and then the screensaver kicks on in another 2 min (3 min after last input).
If the other fixes don't work for you, maybe this one will. Also, I've read that sometimes installing xscreensaver trips something in the KDE screensaver and it will start working as expected. I haven't tried this though because mine is working and I don't wan't to screw with it any more