Great news! Now that the contract is up with AT&T Apple can finally sell unlocked iPhones here in the US. The only 2 supported carriers in the US are AT&T and T-Mobile. T-Mobile does not use the same 3G frequency as AT&T so those looking to use an unlocked iPhone 4 on T-Mobile will be limited to edge speeds.
Now that you can get an unlocked iPhone in the US you can finally take your iPhone overseas to use on a different carrier. Pricing starts off at $649 for a 16GB iPhone 4 and this is a contract free iPhone. No word yet as to whether or not current AT&T customers can get their previously purchased iPhones unlocked. Hopefully you can call up AT&T and tell them your IMEI code and they will unlock it for free of for a small fee.