lurker: Only just saw this topic but we had the same with new HP Elitedesk 800 G1s, DP port monitor would go black but PC still active. Had to reboot each time.
Updated everything and haven't seen it since. Out of a batch of about 16 only had 3 doing it regularly though, so was a weird issue.
I had this issue as well, I couldn't get it fixed, until I upgraded to Windows 8 and since then not a single issue.