Sleepy Mac Pro in Mountain Lion
I too have had this problem from time to time with my Mac Pro since Mountain Lion, along with some strange permission build-ups (5+ pages associated with my Epson R3000 printer) since the change to ML. In my case it happens sporadically. Most often I have seen this issue (failing to wake from sleep) after allowing the system to go to sleep while having an app up e.g. Photoshop, Aperture, Lightroom, etc.. Since I now keep my printer off unless I need it I haven't retested this issue since yesterday's Epson Printer driver release by Apple. I'll see.
But unfortunately I don't have a clue as to why the sleep is hapenning. Hopefully future updates will address same.