With 3.17-rc1 DPMS stopped working. I've bisected this to d55b4af909bc16f7982c2b8b8656f0898158627b testing along the way with xset dpms force off. Controller is:
NVIDIA Corporation G86 [Quadro NVS 290] (rev a1)
That commit doesn't revert cleanly. I took a look around and found a couple of bugs that made it sound like DPMS issues with DP have been ongoing for a while, but this is a DVI connection in my case (to a single Dell 2405FPW). Then I found bug 82527, which mentioned the problem exactly.
My experience with log capture around the xset dpms force off command is exactly the same as Martin's in that bug. No additional output.
Prior to d55b4af909bc16f7982c2b8b8656f0898158627b I didn't have any serious problems (the immediate switching back on of the screen after a xset dpms force off the first time not withstanding, I've lived with that for years).