A number of users have reported significantly shorter battery life under iPhone software/firmware 1.1.3. It appears that the update makes changes to power management/delivery, or introduces application-specific issues for some users that can cause inordinate battery drain.
Our special report on fixing short iPhone battery life contains a number of tips for reducing power usage, eliminating corruption and performing other procedures that can restore normal battery lifespan.
One fix that has proven especially effective for users experiencing this issue after the 1.1.3 update: Force-quit the Mail application every time you are done using it. This can be accomplished by holding down the Home button for several seconds while in the Mail application until the home screen reappears.
It seems that a flaw in the Mail application causes it to persistently generate server activity even when it is not checking mail per user settings.
If this workaround appears to solve your battery issues, you may want to consider deleting then re-establishing all email accounts per our special report.
Persistent mail checking can quickly drain iPhone batteries. Unfortunately, even if you change the automatic check mechanism (Settings, choose Mail then tap Auto-Check) the preferences may not stick. Several readers have had success extending battery life by deleting then re-establishing their accounts.