Finally Dev Team Release the new Pwnage tool 3.1.4 for MAC, which helps you to Jailbreak your iphone with Firmware 3.1.2, read the tutorial here. Now after the Jailbreak so have to unlock your iphone to use it any of the carriers.
Here is the Guide :-
Unlock iPhone 3.1.2 with ultrasn0w | TechnoBuzz.net
Here is the Guide :-
Unlock iPhone 3.1.2 with ultrasn0w | TechnoBuzz.net