I faced this problem over the weekend myself but I've just received reports from other Nokia Lumia 800 users which had the same problem recently. As a matter of fact, my Lumia 800 didn't turn-on last Sunday morning after I turned if off Saturday night! When I switched if off, the battery had around 1/3 of its capacity so it wasn't drained. Nevertheless, nothing helped, doesn't matter how long or hard I pressed the power button. It only came back to live after I connected the device to my PC and Zune kicked-in after a couple of minutes and confirmed with the typical Windows 7 sound that the PC connected to a USB device. Later, the Lumia 800 showed the red battery icon, indicating that the device was charged.
Other users reported that it took them over an hour to wake it up by removing the SIM card and keeping the power button pressed for over 10 seconds. Another user did the same as I did by plugging the Lumia 800 into a PC and letting Zune kicking-in to reawake the device.
We know and have received the confirmation, that the Lumia 800 has a battery drain problem but that's the first reports I've heard, with three received confirmations of other Lumia 800 users, which have faced the same problem as I've seen. Have you noticed anything similar and were you able to reproduce it?
Cheers ~ Arne