So I did some experimenting today and found out why my 3G iPhone was losing battery power in about six hours.
Problem? The sim. It is a sim card from a Samsung cheap little flip pay as you go phone from AT&T. The sim will acivate the iPhone with no problems through iTunes after restoring, that I was surprised about. It's just the account associated with the sim has no data plan so the phone can only make calls or access data while on Wifi. No problems there. The problem comes in to affect when the phone is sleeping and the phone switches to edge or 3G. That drains the power very quckly as it can't connect to get a data connection.
Easy fix: Airplane mode or remove the sim.
I verified all of the above by placeing that sim in my 1st generation iPhone and the battry drained 50% in just two hours. I put the phone in airplane mode and the battery has stayed on 20% for the past hour.
My 3GS phone is also now Jailbroken using Purplera1n and of course since that phone has a real iPhone sim in it with a dataplan, that one never had battery draining problems.
Just putting the idea out there because I know many people here aren't using AT&T sims in their iPhones and some have no data plan. Your battery issues may be caused by this.