Hey... I'd encounter similar issue (iPad2 got shutdown) this morning. Was on my bed watching video via AVplayerHD last night. Just before turning in for the night, I 'close' the video app with home button, and put it to standby mode.
Today morning was going to read some news with my ipad, screen remain blank upon pressing the home button several times. Standby button was not responsive too. I assume the ipad was shutdown, so press and hold the Standby button for a few more seconds. Was relieved after Apple logo starts to show up. Battery level was around 80% last night, 58% after the Standby screen shows up.
I have been an iPhone user for more than 3 years, never encounter any unexpected shutdown on my iPhone3G or iPhone3GS before. Any reason causing the ipad to shutdown? By the way, my iPad2 is currently running on iOS 4.3.5