(I'm not sure this is a g-p-m problem, but I'm starting there since it is a power management issue. :))
I reinstalled my machine with Fedora 11 (used to be Fedora 10). After that, the monitor no longer suspends when I lock the screen. I've set 30 minutes in g-p-m and it works fine if I just leave the desktop as-is. But as soon as it is locked, the PM settings are ignored.
Works fine for me in F12.