A SIM card hack known as Gevey has been released to unlock the iPhone 4 running the latest iOS 4.3 software and baseband. This is a hardware hack as opposed to a software hack like Ultrasn0w.
The hack involves a wafer thing SIM, that needs to be inserted in the SIM card tray along with the actual SIM card that you wish to use in your iPhone 4. You then have to follow a short procedure of dialling an emergency number, hanging up, then toggling flight mode on and off a few times. Then service with your chosen provider magically appears. It is a little convoluted and has to be repeated if you reset your iPhone 4 or run out of battery.
The Grevey SIM will cost in the region of $75 and supports iOS 4.0, 4.1, 4.2, 4.2.1, 4.3 and baseband 1.59.00, 2.10.04, 3.10.01, 04.10.01. This is a costly solution if you are in need of a SIM unlock for your iPhone 4. It is currently the only one available; and you do not need to jailbreak! If you really need an unlock solution, it may be worth considering.