This method is only for those who are using an unlocked phone on the 3.1.2 firmware. As mentioned in the previous post, I bought my iPhone 3GS from Starhub Singapore and its already unlocked. I was able to use it with my Thai AIS Sim card. So, it must be unlocked. Also note that you should only jailbreak with BlackRa1n if you are on the Firmware version 3.1.2.
Step 1: Make sure that you have the latest version of iTunes. I was on iTunes 9.
Step 2: Download BlackRa1n from www.blackra1n.com
Step 3: Plug your iPhone to your computer. It will now auto launch your iTunes and search for a new firmware update. You can cancel that and close iTunes. Launch BlackRa1n. Click on “make it ra1n”.

Step 4: Your iPhone will now enter recovery mode. The regular recovery mode image on your iPhone will be replaced by a picture of GeoHot.


Step 5: BlackRa1n will run on your iPhone and then it will reboot.

Step 6: Once your iPhone reboots, you will notice a new icon on your springboard. This is BlackRa1n. Now make sure you have internet connection on your iPhone and tap on the icon below.

Step 7: Install Cydia. This is like the App Store of your original iPhone except that it has many programs not available in App Store such as the Winterboard etc which allows you to modify the theme in your iphone. You can install all 3 if you like. I installed Cydia and Rock.

Step 8: BlackRa1n will download and install the selected applications. Once done installing, your iPhone will respring and you should see your installer app on your springboard. In my case will be Cydia and Rock.
Step 9: Once your installer application is installed. You can safely uninstall BlackRa1n by launching the app and selecting “Uninstall blackra1n”.

Step 10: Reboot your iPhone. Enjoy!
Note that when you reboot, you will see the screen which asks you to connnect your usb to your iPhone. And when you connect, it will seem to restart what you have done all over again. iTunes will be launched and searching for firmware update. DO NOT do the restore. You can just cancel it and run the Blackra1n application again.
This is the only problem we have at the moment. Everytime you reboot your iPhone, you will need to connect to the PC and run the Blackra1n application to boot up the iPhone. Personally, I dont think there is anything to worry about. Just make sure you charge your battery every night and do not install applications on your iPhone when you do not have a computer.
This method works for me. Pls note that I will not be responsible for any problems arising from this tutorial. I am posting from experience. Jailbreak it and have fun. Feel free to comment!!