New day, new tweak in Cydia: Fimrware 3.1.3. This tweak changes your reported firmware from 3.1.2 to 3.1.3 and tricks app store apps and iTunes so you don’t get prompted to update your firmware.
FSM EXCLUSIVE DEAL This is not really iPhone firmware 3.1.3. It is just to trick your device into thinking you are updated by updating the file at /System/Library/CoreServices/SystemVersion.plist. This will only work if you are on 3.1.2 since it is virtually the same as 3.1.3. Uninstall the package to reverse to firmware 3.1.2.
The tweak is available for free in Cydia via BigBoss repo.