I have a late 2011 MBP running Lion 10.7.4. When I have it plugged into the wall and I close the lid, it goes to sleep. But when it's on the battery and I close the lid, it does not go to sleep. Why is that?
I found out it was an app called NoSleep that was the cause. I had installed it months ago, but thought it wasn't running because I unchecked the "Start NoSleep utility on system startup" setting. But apparently that only controls the menu bar icon. NoSleep is actually always running, and it was set to prevent sleeping on battery.