iOS 4.0.1 has been released for iPhone 4, 3GS and 3G. While the iPhone 4 and 3GS users will have to wait for a new jailbreak tool, iPhone 3G users who doesn’t rely on an unlock can jailbreak their phone on iOS 4.0.1 using Redsn0w 0.9.5b5-5.
Again, this is strictly for iPhone 3G only. If you rely on ultrasn0w carrier unlock, DO NOT update to iOS 4.0.1 until further notice.
Step 1: Download iOS 4.0.1 for your version of iPhone. Download links are posted here.
Step 2: Update to this latest firmware using the IPSW file you just downloaded via iTunes 9.2.
Step 3: Start Redsn0w 0.9.5b5-5 (download link given below) and point it to iOS 4 (Final Version), and not iOS 4.0.1 firmware file to proceed with the jailbreak.
Step 4: Now select “Install Cydia” and click “Next”.
Step 5: Now make sure your device is both OFF and PLUGGED IN to the computer before you click “Next”.
Step 6: Connect your device in recovery mode by pressing the Power and buttons continuously.
At this stage you will be presented with a screen where you will have to quickly hold “Power” button and then while holding the “Power” button, you will now have to hold “Home” button too. Now after few seconds release “Power” button but keep holding “Home” button until installation begins.
Step 7: Sit back and enjoy while redsn0w will do all the rest for you. Once done, your iPhone 3G will restart automatically and you will find it jailbroken on the latest iOS 4.0.1 firmware.
Disclaimer: This guide is for testing & educational purposes only. I’m not responsible for any loss of important data or malfunctioning of your iPhone.