I recently discovered that leaving the unit in Alt-aA mode and leaving it with its buttons pressed (like the embrace from a bag's zipped net pouch) will drain the battery in a few hours.
Leaving the wireless on can drain the battery very rapidly if the Kindle winds up in a wireless dead zone. The unit doesn't seem to be incredibly clever about backing off on trying to reach the network to save the battery.
I've also noticed that large books eat the battery like candy as the Kindle works to index the book after download. Purchase several tomes at the same time and you can lose a bar off the meter over the next hour even with the unit otherwise completely idle.
Every time I flattened my Kindle's battery (testing the buttons) I had to reset it to recover it. Having designed portable battery operated devices myself I can attest that a solid self-reset from a battery drain with Intel CPU's is tough. They just seem to need an extra whack to get them to start working again.