Mas de uno seguramente tuvo mas rapida la mano que la mente y le dio el OK a la nueva actualizacion del Firmware 4.0.2 con el cual no podemos hacer el Jailbreak o Unlock de nuestros 3GS.
Con la siguiente info podemos hacer el Downgrade sin la necesidad de haber salvado en su momento las SHSH!!!!
La info esta en ingles, no voy a usar el Google para traducir ya que los pasos son simples y si no, aprendan!!!
Step 1: Download the iOS 4.0.1 from here, download iTunes from here.
Step 2: Extract it with Winrar or Winzip to a folder on the desktop. You may need to rename the firmware file from .ipsw to .zip to be able to extract.
Step 3: Open buildmanifest.plist file from the extracted files using Notepad (for Windows) or TextEdit (for Mac OS X) Search and replace all – 8A306 with 8A400. Do the same with restore.plist file and save both of them.
Step 4: Now download iOS 4.0.2 and extract it using Winrar or Winzip.
Step 5: Drag all files extracted files from iOS 4.0.1 and replace them with all files of iOS 4.0.2.
Step 6: Delete all ".dmg" files that end with 002 leaving files that end with 001. Then zip the new customized firmware and change it to ".ipsw" file, for example "downgrade.ipsw".
Step 7: This step is option, it helps you to ensure you get an SHSH file request for the future, but should not be necessary to just restore 4.0.1
Locate "C:WindowsSystem32driversetc" for Windows and "/etc/" directory for Mac, then locate “hosts” file and Open it in Notepad (for Windows) or TextEdit (for Mac OS X) then add this line at the bottom of the file and save it.
Step 8: Connect your iPhone to your PC / Mac and launch iTunes (downloaded above)
Step 9: Now, put your iPad in DFU mode by following this steps,
Connect your iPad to your computer.
Turn iPad off.
Hold Power and Home buttons together for 10 seconds or so.
Release Power button but keep holding the Home button until your computer recognizes a new USB device.
iTunes will now recognize your iPad.
Step 10: Almost done, you will have to restore your iPhone to the firmware you have customized and saved in the above steps by Clicking on iPhone icon that appears in the left of iTunes then hold left “alt” button or “Shift” button on Windows from the keyboard and click on “Restore” then release the alt button. You will be promoted by a window to select the firmware, select your firmware file "downgrade.ipsw" you have made in the previous steps.
Step 11: After series of processes, iTunes will restore your device to the required firmware.
Now, You will be finally able to jailbreak and unlock your device using our guide posted here to jailbreak and unlock any iPhone.
Update 1: While downgrading from iOS 4.0.2 to iOS 4.0.1 / 4.0, users may face error 1015 in iTunes, use Error 1015 Fixer (download here) to bypass from this error.
Update 2: Here's the modified .ipsw file that you should restore to for iPhone 3GS users It is a 4.0.2 firmware bundle, with the 4.0.1 files inside (as instructed) After restoring you will be at 4.0.1. it can be downloaded from here (http://db.tt/T5MjgPF).