If you are wanting to unlock iOS 4.2.1 on iPhone 3GS & 3G with Ultrasn0w 2.1, then follow the instruction below:
1. Launch Cydia via your iPhone.
2. Tap on Manage tab, then tap on Source, and add the source.
3. Then tap on Edit, Add the following url source http://repo666.ultrasn0w.com, then continue tapping on Add Source:
4. As usual, you will see the automatic update for your source by Cydia, just wait the installation is finished. After it's done, search ultrasn0w 1.2 within Cydia in order to install this application. After it's installed, your iOS device will be unlocked automatically, as the result you can use it using any carrier of your wish.
5. Then complete the steps by restarting your iPhone to have iPhone 3GS (or 3G) unlocked on iOS 4.2.1 and congratulation!
See you again!