international i-phones unlocking

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i didnt realize that there was a difference between i-phones till i was looking at the latest i-phone 4 at the apple website. i always thot that unlocking the i phone was like the usual key pad lock n unlock feature until i discovered that apple i -phones are registered to AT and T and cannot use applications apart from apple's unless it is jail break and unlocked. so to use your local sim card the i phone needs to be unlocked.so can i use a 2nd hand i-phone from the US n how to unlock it for international users?