So you updated your iPhone 3G to 4.0 but now want to unlock your iPhone to use with a different carrier. Not a problem. First you must Jailbreak your iPhone 3G on 4.0 if you have not done so, check out our guide here. You don't have to have windows or osx since this is all done from your iPhone. Ok this is actually pretty easy should take less then 5 minutes.

1. Start Cydia and let it update it self.
2. Click Search and type in ultrasn0w
3. Top right corner click install, then confirm
4. Once installed click Reboot.

Viola thats it, you are now carrier unlocked and can use which ever sim card you want.