battery life in 3g iPhone is short despite battery change

I recently replaced a battery in my iPhone 3g because it was dying in a matter of a few hours. The new battery is holding its charge but the phone is still dying before 5-6 hours of use. Networking, including the cellular networking is turned off. I did not do a full system reset following installation of the new battery, which went very well. I'm wondering if the phone was hacked and is running something in the background that can't be detected from the usage meters.