Though Apple has officially launched unlocked iPhone 4 in 17 countries including Australia, Canada and Hong Kong, there are many other owners of iPhone 4 in the United States who are forcibly bound to a contract in order to use iPhone 4. But gone are the days of monopoly. With this new application you can just about any carrier you want for you iPhone.
The first thing you need to do is to Jailbreak your iPhone 4. Once you have Jailbreaked, install Cydia. Cydia is just like Apple iTunes where you can download applications for your phone. Once you have installed Cydia, add ultrasn0w 1.0.1. You will find this in Cydia->Manage->Sources. Once added there will be a prompt. Enter the following in the pop up window - http://repo666.ultrasn0w.com
And enjoy an unlocked iPhone. This application will work on following models;
iPhone 4 with baseband of 01.59 and iPhone 3G/3GS with basebands of 04.26.08, 05.11.07, 05.12.01 and 05.13.04