Originally Posted by danix
If you could try running a bare X server and/or killing gnome-power-manager (if you're even running it in the first place) to see if the problem goes away, it might give us a clue that your problem (which we haven't reproduced) is or isn't related to the other one which we're investigating (and have reproduced).
I'm not running gnome-power-manager. Right now I'm using KDE 4.6.3 (which, afaik, uses powerdevil+upower for power management).
I'll still try a bare X server (some time tomorrow, hopefully) to see if that makes a difference.
If it's any help, the DP->DVI monitor shows the same behaviour when I switch to a VT. I.e. the primary monitor (DVI only) stays on, while the secondary DP->DVI monitor flickers on and off.