Ok guys I just tested it on an old iPhone 3G lying around and can confirm that this works! Now that Apple has released iOS 4.0.2 to patch the PDF exploit, JailbreakMe which was previously used to jailbreak iOS 4.0.1 on iPhone 4, 3GS, 3G, iPod touch 3G / 2G and iPad is now incompatible with iOS 4.0.2. But if you have iPhone 3G (which doesn’t rely on an unlock) or an iPod touch 2G (non-MC model) lying around, you can jailbreak it using the existing version of Redsn0w. Simply follow the step by step guide posted below to jailbreak the mentioned devices on iOS 4.0.2 using Redsn0w.
Step 1: Download iOS 4.0.2 for your version of iPhone and iPod touch.
Step 2: Update to this latest firmware using the IPSW file you just downloaded via iTunes.
Step 3: Start Redsn0w 0.9.5b5-5 and point it to iOS 4 (Final Version), and not iOS 4.0.2 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 and/or iPod touch will restart automatically and you will find it jailbroken on the latest iOS 4.0.2 firmware.