Had the same happen on my SGS2 last night, listening to music via Bluetooth, and have done this many times before without problems, but last night went from full battery to 30% in 90 minutes.
I have experienced this before with another Android device (Sony X8) the circuitry (not the battery) gets very hot and the battery appears to drain rapidly. Im pretty sure its not the battery itself but more the phone doing something.
To cure this I removed the battery and replaced it, I then charged it with my emergency Duracell charger and it then ran cool and ok for the remainder of the night (im a night shift worker).
I believe by taking the battery out and replacing it, you actually force a hard reset of all circuits and components in the phone, and it seems to stop the overheat/rapid discharge problem. This also seemed to work with the x8.
It has nothing to do with the casings, the x8 would do it without any additional case, my S2 is otterbox defender protected. My personal opinion is that this is a "feature" of the Android software, and not a hardware problem.