Auto3G jailbreak app. If you're tired of your iPhone battery life performance and need to extend its life longer, then there is a jailbreak app listed on Cydia, named Auto3G that can do the job. Auto3G is designed to extend battery life of iPhone 3G, 3GS and iPhone 4G.
This is the overall how Auto3G works, it automatically turn off 3G radio when your iPhone is locked and it will turn the 3G radio on automatically by the time you unlock the iPhone. It appears to keep 3G speed without losing battery life perfomance.
You can get this app for $5.99 in Cydia. Some users consider this app expensive, especially when you can easily turn off 3G via SBSettings and can do it anytime anytime, above all, that activity only takes few second to be done.