Can DVI displays and thunderbolt ports be expected to wake the monitors every time from sleep?
I have this same issue on my iMac 27" (Mid 2011) with one external DVI monitor. I took the iMac into the Apple store and the genius couldn't provide an answer to why this was occurring. I also tried using both Thunderbolt ports and the issue would happen on both. So for now what I did was to not let my iMac go into sleep mode. I set it to just have the display's go to black and when I activate the iMac the external display always comes back. Hoping a future update to Mountain Lion OS X 10.8 will fix this issue.