Hey Everyone
I'm one of those who suffered of the same problem last year with my MBP late 2011. I solved the problem that time and wrote about it on this thread, but seems it didn't work to others.
The solution that time was just reinstalling OSX without formatting. That just fixed the issue which was caused by a widget I used to have on OS 10.7 and older. The widget was called "Deepsleep" and it seems that the developer stopped releasing compatible versions for the new Mountain Lion.
Two weeks ago, I wanted to use that widget, so I went to its official website, downloaded the latest version there and installed it. Then the same sleep/hibernation problem was back.
I uninstalled the widget cos I realised that it was the reason for sure and reinstalled Mac OS again, but this time it didn't solve anything. I desperately headed to the Deepsleep's website and searched the support page there and found out a hint to return the a default setting that was causing the whole problem for me.
According to a PDF I downloaded from there I entered the following on Terminal:
pmset -g | grep hibernatemode
That is to check the "hibernatemode" value on the machine
then
sudo pmset -a hibernatemode X
with X replaced by the value of the hibernate mode to set. My choice was 3 according to a page I found online http://etherealmind.com/osx-hibernate-mode/
You can find even more result if you just Google this "mac hibernatemode desktop laptop"
I've been using my Mac normally and closing the lid to sleep several times a day for more than a week, and it's waking up just normally as I open it. I even happen to sleep it while the battery was lower than 5%, the Mac in that case went to hibernation mode after a while and woke up successfully.
Hope that works for you all.
Ayham