I was able to solve this problem on my Galaxy S 2! Overnight my battery life dropped to maybe 3-4 hours (on idle) and it became completely unusable... I don't run any programs that drain battery life and GPS/WIFI/Bluetooth are always off, so I was really confused.
All I did was follow a recommendation (from this thread I think) to shut off the phone and take out the battery. Then I put the battery back in after a minute. When I turned on the phone my battery life suddently dropped by 15% but then started draining at a normal rate!
Not sure if this will work for everyone but its easy to try.