Finally after months of hard work and trials there are solutions to unlock your Apple iPhone 3G models. Here are the best links for the same :
Tutorial to Unlock Apple iPhone 3G
Hack Apple iPhone 3G

This unlock is done through the tool named yellowsn0w which can be installed through the Cydia installer. Having Cydia in your iPhone would need the latest version of firmware i.e. v 2.2 whose installation will let you install Cydia and download yellowsn0w which will unlock your iPhone for every network.

The unlocking also requires you to reset your network settings and then play with the airplane mode, before installing your 3rd party SIM. QuickPWN tool would first jailbreak your phone, then help in upgrading the firmware and installing the tool developed by iPhone Dev Team. It was tested successfully for U.S. network At&T and indian carrier Airtel.