I'm trying to buy an iPhone on craigslist and then unlock it. What is the latest iPhone OS/firmware combo that I can unlock the phone with? I know that 3.1.3 and Firmware 5... is a no go, but 3.1.2 with Firmware 5... is good? How about 3.1.3 with old Firmware 3...., can I downgrade in that case? Thanks.