I have a Mac Pro Mid 2010 8-core running 10.7.1. I routinely have to press the power button twice on startup. The first time I press it, I hear a click but no power up. Approximately 5 seconds later I hear a second click. After this second click I can press the power button and the system powers up normally.
I searched and saw that someone had pointed to not having a startup disk selected in the Startup Disk Preference as a possible cause. I do have a startup disk selected.
I have an ATI Radeon 5870 graphics card upgrade, but the card works correctly and I have re-verified that I have all the proper connections.
I have followed the instructions for the resetting the system management controller, but no change. Unfortunately, I do not know when having to press the power button twice started, so I can't trace to any particular system or software configuration change.