I had a really weird problem when using a custom ringtone.
I created a "ringtones" dir on the SD card and set my default ringer to an .mp3 file in that dir.
The very next day, even after charging all night, my battery was draining extremely fast.
Long story short, I deleted the ringtones folder, went back to using a default ringer that came with the device and everything was back to normal.
My theory is that since I was using a ringtone on the SD card, the phone kept the SD card powered which didn't allow it to sleep.
This seemed to be evidenced by the fact that after assigning the custom ringtone, Android System was #2 in my battery list. (cell standby was #1)
Before assigning the custom ringtone, and after removing the custom ringtone, Phone Idle once again became #2 in battery usage.