Firmware Changer Lets You Change Your Firmware Version to iOS 4.1 Without Updating

If you care about changing your iOS version visually on your iPhone for some reason, then you can useFirmware Changer instead of editing .plist files and making a complicated process. Firmware Changer will not let you update to iOS 4.1 but it will fake to be on iOS 4.1 or any other version.

You may want to change your firmware changer so that iTunes doesn't annoy you with the update message or may be to install some app that works only on iOS 4.1. Anyway, Firmware Changer is a free tweak available in Cydia via ModMyi repository.