I was using my edge several of hours ago and needed to shut it down real quick, so I hit the shutdown button and it brought up the shutdown and silent mode options. I ended up closing the edge before making the option of shutting down. So my EE sat for about 2 hours and then I went to turn it on. I hit the power button then proceeded to open the edge only to find that it was hibernating. Just out of curiosity I hit the menu switch and it resumed my session
So from then to now I've been trying to find out what I did to trigger this event, and here are my findings:
Attempt 1: Brought up the power option screen, closed for 20 minutes, hit the power button, opened and hit the menu button ... success
Attempt 2: Did not bring up the power option screen, closed for 15 minutes, hit the power button, opened and hit the menu button ... failure
Attempt 3: Brought up the power option screen, closed for 40 minutes, hit the power button, opened and hit the menu button ... success
Attempt 4: Brought up the power option screen, closed for 50 minutes, did not hit the power button, opened and hit the menu button ... failure, then tried to close it again, hit the power button, open it, and wake it up and ... failure again.
Now I'm going to Bring up the power option screen, close overnight, hit the power button, open and hit the menu button, and figure out once and for all if this is legit for me. [update] I could not get it to resume. I'm suspecting that it will not resume the second time you try to hibernate it as glenj said was normal. I'll try a couple more times to see if I can wake it up in succession, but it seems to me that I jumped the gun.
Please feel free to try and see if this works for your unit (the bringing up the power options and hitting the power button before trying to resume). It would be nice to fix this issue, and if this work-around works, maybe someone smarter than me can figure out a permanent fix.
(also to note, the device was not charging during any of these experiments)
[edit] scroll on down to comment #21 for a nice work around for the hibernation issue and the discussion about it afterwards.