George Hotz, released the first jailbreaking app for IPhone titled ‘purplera1n’. The app works only on Windows Platform (Windows 7 unsupported) now supporting Windows 7 and 64 bit seamless work with latest version of iTunes, with iPhone 3GS with 3.0 firmware and now also available for mac download it here.

The process is simple as described on the Developer’s blog:

Connect your iPhone normally. Click “make it ra1n”. Wait. On bootup, run Freeze, the purplera1n installer app. Hopefully you’ll figure out what to do from there.

Please unsure that you backup your data and try at your own risk. and more details how to unlock the iPhone 3GS are on his blog “On the iPhone”