If you have jailbroken since you made the backup, your firmware is not the same whatever the number in Settings>General>About. That's the whole point of the jailbreak...it fundamentally alters the OS.
Wirelessly posted (iPhone 3GS (JB3.1, unlocked): Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7C144 Safari/528.16)