I have a 2008 Mac Pro, and when I upgraded to Mountain Lion this week, my system no longer wants to sleep. I select sleep from the apple menu, and the screen turns off, but I never hear the hard drive, fans, and power light do their normal sleep thing like they used to. The fans are still on and the light is still steady, it's not doing the slow flash telling me it's sleeping. Nothing in my system has changed with the exception of installing mountain lion, 10.7.5. Only USB devices I have plugged in the the keyboard and mouse. No other hard drives installed, and the NVIDIA GeForce GTX 285 is the only expansion card I have, and it's been in there for years. I have reset the PRAM but it still just sits there.
I checked the hibernate mode and it's set at " 0 ".