Mountain Lion aggressively hibernates
I noticed the same. Luckily I had the free support option still active and I contacted them. See my post here at Apple Support Communities: MacBook Pro wakes from sleep at connect/disconnect AC power.
Regarding waking up when AC power is attached/detached I have contacted Apple support and they were able to reproduce the problem. They consulted the behaviour with their engineers and they said that this is the expected behaviour. I don't know what is the reasoning behind this however I was also informed that I can provide feedback (via apple.com/feedback ) and I did. If you guys think (like I do) that this feature is annoying and probably dangerous (when you detach the sleeping macbook from power and start toss it to put into backpack while hdd spins...) then I suggest to nag Apple on this matter.
Basically, they admitted that the behavior is 'not a bug but a feature.'
ML 10.8.2, mid-2012 15" MBP (non-Retina).
This problem just popped up for me a couple weeks ago: leave MBP sleeping for X hours, unplug MBP, MBP lights/wakes up, enters hibernation. Really annoying to open the lid only to have it shut down right in front of me.
My acwake was also disabled for both battery and AC. Which means that the setting is now being ignored.
The solution appears to be (even though it's not referenced in the pmset man page):
sudo pmset -a autopoweroff 0
I should note that this does not affect acwake being ignored. My computer still wakes up upon AC connect/disconnect, but it's not hibernating or attempting to hibernate after I've unplugged the computer and opened the lid.
To control exactly when your Mac will transition from sleep to hibernate, you can use the following (the delay is in seconds) and you need to have hibernate mode to be set to either 3 or 25 (binary 0011 or 0001 1001):
pmset -a standby 1 standbydelay 600
You could use different settings and timings for -b battery and -c charger if desired. (Perhaps on power, you disable standby mode, but on battery have a 10 hour timeout.) These switches are documented in the manual page for pmset. I haven't messed with acwake lately, but in the past the lid proximity sensor of a portable would take precedence and suppress a wake that would have happened if the lid were open.