I was just having this issue myself and after going through a ton of thread I was able to solve my issue.
I installed Snow Leopard 10.6.0 and did the combo update to 10.6.2 and started having monitor issues. The display would not wake after sleep, and on reboot the display would still not awaken. I could still connect with Screen Sharing. The display would eventually start if I unplugged the display power and the connection to the video card and once the Mac Pro booted into 10.6.2 I could reconnect and the display would turn on, sometimes.
If you have similar issues what seemed to work for me was a SMC reset,
http://support.apple.com/kb/HT3964
This resets the management control and seems to get the display to start responding again.
To reset the SMC:
1. Power Off The Mac Pro
2. Unplug the power from the back of the machine
3. Hold down the power button for 5 seconds
4. Plug in the power.
5. Hit the power button and it should work.
I hope this works for some people that may be having this issue.
Good luck.