How to Unlock iPhone?

"By default" Apple sells the iPhone locked to AT&T operator. So, to use iPhone "as it is" (locked), you’ll need to sign up for a 2-year service agreement with AT&T. Are you sure you need it?
There are many sites dedicated to this problem, so unlocking an iPhone will be easy.

For example, you can jailbreak and unlock almost any iPhone instantly with ZiPhone.
iPhoneSimFree.com - use any SIM with your iPhone!
www.hackthatphone.com - an international site featuring instructions for modding the iPhone.
iPhone Jailbreak hack - latest Mac version.
iPhone Jailbreak hack for Windows - latest Windows version.
iPhone Hacking Guide - all you need to know about hacking and unlocking.

Also you can buy cheapest unlocked iPhone and iPhone unlocking software on eBay auction.