Seems to be an issue with Dell monitors - I get a similar problem with my u2711 and an HD5870, using DP.
When i start up the keyboard/mouse lights up, and the monitor goes into 'active' mode without any display and stays there for ages, not going into 'standby' mode.
Mines watercooled so dont hear the fan, but if I unplug the monitor power and plug it back in, the display comes on and its fine.
Wierd.