Unfortunately you can't unlock that phone, because your basband is not downgradeable. There is apparently an unlock coming for it soon, so jailbreak it for now with Redsnow and wait for the unlock.
my iphone 3g home button is broken and i need to jailbreak it to get an sbsetting mod to get my home button on the main screen, how can i get a program to jailbreak it without a home button, any firmware 4.0, 4.0.1 and 4.1, or even 3.1.2