Some users (me included) have experienced the failure to wake from sleep problem exactly as you describe since some point in Snow Leopard 10.6.x, through Lion 10.7 and into MLion 10.8. For me the problem went away in 10.8.2 but I have seen the occasional thread on here describing the same problem. It seems to be connected with Bluetooth failing to fully wake up.
The 'fix' that worked for me - actually a workaround rather than a fix - was: System Preferences>Energy Saver and set 'computer sleep' to 'never'.
There one huge thread on here discussing the problem and there have been quite a few others as well.
The display will sleep but the Mac won't hibernate. The Mac should spring to life immediately on touching keyboard/mouse, etc. I know there are some on here that have it set this way as a preference, with no ill-effects on their Macs.