Just found iOS 4.0.1 Custom Firmware for iPhone 3GS Old Bootrom to Jailbreak iPhone 3GS with the Easy way without need to launch Sn0wbreeze or PwnageTool and create your own Custom Firmware, Here's a ready Custom Firmware, all you have to do is to restore with this Firmware using iTunes..
Your Device must be pre-jailbroken on iOS 4, If not then you can use use PwnageTool on Mac or Sn0wbreeze on Windows to Jailbreak iOS 4 on iPhone 3GS Old Bootrom.
Step 1: Download the Custom iOS 4.0.1 Firmware and iTunes 9.2.1 [Links at the end of page]
Step 2: Connect your Device to Computer then launch iTunes
Step 3: an iPhone icon will appear on the left, Click on it
Step 4: Now hold left “Shift” button (“Alt” button on Mac) and Click on Restore, If it doesn't work then try going in DFU Mode or Recovery Mode.
After Successfully Jailbreaking iPhone 3GS, You will be able to Unlock it using Ultrasn0w 0.93, Just follow the step by step guide posted here.
Download iOS 4.0.1 Custom Firmware for iPhone 3GS Old Bootrom
Download iTunes 9.2.1 for Windows and Mac