Let it drain/die and then put it on the charger until it's full.
Could be either:
- iOS 5 Battery issues (should be fixed in 5.1/5.0.2 as they're working on it).
- Battery calibration being buggy. Kill it and reacharge completely and it should recalibrate.
- Some people have reported that after iOS 5 they had this, but doing a restore as a "new" phone fixed it. Possibly something carrying over from iOS 4 being buggy in iOS 5.
This has happened to me before. Battery dropped quickly, phone was warm and usage and standby times were the same. After fiddling around and restoring, these two things stopped it, turning off iCloud backup backup (if you use this, do it manually, then turn it off after it backs up). Second, Reset All Settings, not content or data, just settings. That will be almost as good as a restore, especially if you restore from backup.