My HTC Thunderbolt was powering itself off when the internal battery monitor showed a 50% charge, so I ran a google search. One "Tip" suggested removing the battery pack and checking it for bulges. Sure enough, the battery pack is bulging. It went on to suggest that a cell inside the battery has exploded, and that I should not place it back in my device.
I'm not sure what to do from this point. The battery is model BTR6400B.