Now you can unlock your iPhone 3G and can use it with any mobile service provider.iPhone Dev-team had announced that the “Yellowsn0w” ,the iPhone 3G soft Unlock tool ,is now available for download .
Yellowsn0wis the world’s first software unlock for the iPhone 3G. With yellowsn0w you can unlock your iPhone 3G firmware 2.2 .
Here is a quick guide to unlock the iPhone 3G:
You need to install the latest 2.2 firmware (downloaded from here).
Jailbreak your iPhone, using QuickPWN 2.2. Those using MacOS 10.5.6 need to re-enable DFU mode, which Apple disabled as a way to prevent Mac owners from jailbreaking their iPhones.
You can install yellowsn0w 3G unlock by using either Cydia or Installer, add the following addresses to add yellowsn0w to your installer application:
For Cydia enter: http://apt9.yellowsn0w.com/
For Installer enter: http://i.yellowsn0w.com/
(source)For details refer http://www.crunchgear.com/2009/01/01/how-to-unlock-your-3g-iphone/