If you rely on Ultrasn0w to unlock your iPhone and so you can use it on any mobile carrier you want, you might be interested to know that the iPhone Dev Team has updated its software to include fixes that can improve standby battery life.
The update also includes tweaks that will improve performance and make your iPhone more stable. Ultrasn0w still supports all iPhones up to iOS 4.0.1 (possibly 4.0.2) with baseband 04.26.08, 05.11.07, 05.12.01 and 05.13.04.
To update, open Cydia and let it download the updates. You should have an option to upgrade.
Thanks to iClarified for the image: