Auto3G is a new jailbreak application that aims at extending your iPhone battery life by automatically turning off the 3G radio when you lock your iPhone, and automatically turning it back on when you slide to unlock.
This simple app allows you to always have the speed of 3G when you need it, without draining your battery when you’re not using it. Auto3G will add a new icon to your springboard from which you can choose the 3G timeout settings.
You can get Auto3G for $5.99 from Cydia. I think it’s a bit expensive but preserving your battery life has a price. Have you tried Auto3G? What do you think?