The new ultrasn0w 1.0-1 software unlock supports iPhone 4 (01.59 baseband), iPhone 3G and iPhone 3GS (basebands 04.26.08, 05.11.07, 05.12.01, and 05.13.04). To run the carrier unlock, users must be on a jailbroken handset and have the Cydia Store app installed. Then, you need to add ultransn0w 1.0-1 from the repository by going to Cydia> Manage> Sources> Add and typing in "http://repo666.ultrasn0w.com" in the pop up window. Once that's added, search for ultrasn0w, install the app, and let your phone restart.
As usual, before doing this remember to backup your device just to be safe. Non-U.S. iPhone buyers can avoid going through all that trouble, however. Apple is currently selling the iPhone 4 unlocked in as much as fourteen countries. This handy info-graphic compares the various out-of-contract prices of the handset around the globe.