We used 'Jailbreakme' to jailbreak the phone. Once it's jailbroken you can go to Cydia (added app. when you jailbreak) and then unlock it for any carrier using 'Redsn0w'. You can now put any GSM sim card in it and it will work. Just don't load the current operating system (4.2) and stay with 4.1 or below. 4.2 is a bit more tricky to jailbreak and unlock. It was designed specifically to stop you from jailbreaking the phone.

The simplest way to do the jailbreaking and unlocking is to go to Youtube and follow one of the videos you find for 'jailbreakme' and 'Redsn0w'. A no brainer to do.

If you want to use it for a USB modem download one of the programs that allow you to do that and you will have internet on your computer.

None of the providers care that your using an unlocked Iphone.