I had the same problem too for a while. It turned out that my xserver was only working on the analog output from my driver. If you have, and are using digital output try and add the following line to the device section of your XF86Config-4 file:
Option "ConnectedMonitor" "DFP"
Caveat emptor: this may only work if you are using a flat panel monitor (DFP stands for digital flat panel).