I have a year old Samsung Droid Charge. In recent weeks the battery dies while the phone is off over night. When turning the phone off it can have a 90% charge, but still dies. I have 2 batteries, one being only 4 months old, and they both die over night.
install an app such as BetterBatteryStats(paid app i think) and have a look at the wake lock. the wake lock will tell you what's keeping it awake. in your case. there will be a massive difference between when the screen is on(note that the display is one of the most, if not, thee biggest user of the battery) and when the phone is awake