While the recent posts we made are about updating to the latest iOS 4.0 GM, we do warn you to try this only if you are a developer and really need to see how the new iOS 4 stacks up against your apps. But if you are a developer and want an early start on optimizing your app for the new iOS 4.0, we decided to help you out by writing out a helpful guide on how to jailbreak the latest iOS 4.0 on iPhone 3GS while preserving your old baseband settings which would allow you to unlock your phone using blacksn0w or ultrasn0w. This guide will patch the existing version of PwnageTool, so that it can jailbreak the latest iOS 4.0 for iPhone 3GS.

Warning: we are not responsible if you end up locking or bricking your iPhone while using this jailbreak method. We recommend that you only implement this technique if you have a thorough knowledge of iPhone Development.

Also, we would like to point out that this jailbreak will only work for the iPhone 3GS with the older bootroms, ones which can jailbreak with PwnageTool, Redsn0w, Blackra1n or Sn0wbreeze method. Bootroms which jailbreaks with the spirit tool are not support this method and will show 1600/1604 error.

• Download PwnageTool 3.1.5.
• Download and extract the latest version of the PwnageTool bundles from msft.guy.
• Download iOS 4.0 GM build from here, and iTunes 9.2 beta (google for the iTunes beta, as I can’t find it, or download from the official site if you are a registered developer)
• Download the updated Cydia bits (data.tar file).
• Copy PwnageTool that you downloaded in Step 1 to your desktop. Right click, and then click on “Show Package Contents”.
• Goto :

Contents>Resources>FirmwareBundles>and paste iPhone2,1_4.0_8A293_bundle.bundle file in this location and then simply close this folder.

• Goto:

Contents>Resources>CustomPackages>CydiaInstaller.bundle> and paste data.tar file (for Cydia) that you downloaded in Step 4.

• Start “Terminal” program on your Mac and run the following commands:

cd desktop

cd PwnageTool.app/Contents/Resources/CustomPackages/CydiaInstaller.bundle

tar -xpvf data.tar

• Close all open windows.
• Now start this patched version of PwnageTool, Select iPhone 3GS as your device

• You will now notice that PwnageTool will automatically detect iOS 4 firmware file. Select this file.
• Click on “No” when PwnageTool asks you “Do you have an iPhone contract that would activate normally through iTunes?”(this is to preserve your older baseband)
• PwnageTool will now create the custom .ipsw file for your iPhone which will be jailbroken. (the “iHAz success” screen indicating the creation of a newly baked custom firmware .ipsw file for your iPhone)
• Once the .ipsw file has been created, you will now have to restore your iPhone to this custom firmware 4.0 that you just cooked using PwnageTool 3.1.5 for your iPhone 3GS.
• Start iTunes 9.2 Beta, Click on your phone icon from the sidebar in iTunes.
• Press and hold left “alt” (option) button on the keyboard and then click on “Restore” button in the iTunes.
• This will make iTunes prompt you to select the location for your custom firmware 4.0. Select that custom .ipsw file and click on “Open”.
• The rest of the restoration is automatically done by iTunes. Be patient and let iTunes do its job (any interruptions in this part of the process can end up bricking your iPhone!).
• Your iPhone screen at this point will be showing a progress bar indicating installation progress.
• After the installation is done, iPhone will restart automatically and you should now have a fully jailbroken iPhone 3GS running on iOS 4.0.

[via redmondpie]