Requires use in service area that has both 3G and 2G (Edge) service. Works on iPhone 3G, 3GS and 4.
Not compatible with iOS 5

Auto3G extends your battery life by automatically turning off the 3G radio (leaving on the 2G radio) when you lock your iPhone and automatically turning on the 3G radio when you unlock your iPhone. You can also have Auto3G automatically turn off the 3G radio when you have a Wifi connection. You can even have Auto3G keep your iPhone on 2G at all times unless specific applications you designate are running. This way you always have the speed of 3G when you need it without the battery loss.

As explained by Apple, this results in up to 100% greater battery life: "7 hours of talk time on 3G, 14 hours of talk time on 2G." (Apple battery info)

Additional features include allowing you to (i) specify applications that if running will prevent the iPhone from switching to 2G after it locks, (ii) specify a timeout interval that will delay Auto3G from switching to 2G after the iPhone locks for the specified interval, and (iii) default the iPhone to 3G while charging even when locked.

Try the sbsettings toggle here

Use the Auto3G icon to change settings