I had this problem, but it doesn't do it now. It turned out to be a calibration issue for me. [...]
Anyway, to recalibrate:
1) Drain the battery until the phone shuts off on its own.
2) Charge it non-stop back to 100% (Should take 2 - 3 hrs)
When I first started draining the phone, I was losing about 5-7% per hour in standby. As the battery drained, the rate the percentage was dropping got slower and when it got to 1%, it sat there for almost an hour. Then it finally shut itself off. I recharged back to 100% in one sitting, and now it's got the same or better battery life my 3GS had that I was upgrading from.
My battery life seemed to improve quite a bit after I had 2 days in a row of draining to 4%, then charging to 100%. of course I did other things at the same time (turn off push, etc.) so hard to say if this 'battery calibration' alone was beneficial.