This morning, a new jailbreak tool ‘Spirit’ is released for the Windows and Mac platform. This tool is said to be able to JB the iPhone 3GS the newest MC131ZA model. In the past, tools only allowed tethered jailbreak, that is if you power down the phone, you will have to JB it using BlackRain again to get it to kickstart the OS. With Spirit, this problem is totally resolved.
1) To jailbreak, you have to download the tools which is presented in the main news page earlier. To begin, you would need your iphone connected to USB. Do a backup if needed.
2) For Windows, right click the icon and set compatibility mode to Windows 98/ME.
4) Once it is done, it will show Jailbreak succeeded!
On the next page are some screen shots of the jail broken iPhone with Cydia installed.