Unlock iPhone 3GS with Firmware 3.1 on Windows

The iPhone Dev team has no plans to release and PwnageTool for Windows users and since there also isn't any ETA on the release of Redsnow, so we have created a Custom/Hacktivated IPSW for iPhone 3GS users who want to update to Firmware 3.1 while preserving the baseband.

Only users with a previously Jailbroken iPhone 3GS (on an old firmware) should attempt to follow this tutorial as it does not work for those who have an iPhone 3GS with Firmware 3.1 out-of-the-box. The tutorial will help to Jailbreak and Unlock your iPhone 3GS on Firmware 3.1.

For the iPhone 3GS

Please use the links to download the custom IPSW of Firmware 3.1 for iPhone 3GS iPhone2,1_3.1_7C144_Custom_Restore.
http://bit.ly/iphone3gs-custom31-sizlopedia
http://bit.ly/iphone3gs-custom31-sizlopedia-p2
http://bit.ly/iphone3gs-custom31-sizlopedia-p3
http://bit.ly/iphone3gs-custom31-sizlopedia-p4
Once downloaded, join the file parts using HJSplit and extract the ZIP archive to have one file. Now follow the steps.

1. Please down and install iTunes 9, and then run it.
2. Connect your iPhone to the PC.
3. Click 'Restore' while holding SHIFT key and select the Custom IPSW.
4. Once everything is processed and OS boots up, install Ultrasnow.
5. Follow this guide (starting from Step-3) to install Ultrasnow on iPhone.

Thats it!
Mac users can make use of the custom IPSW in case they don't plan to get their hands dirty with PwnageTool which, to be honest, is not easy to go by due to lack of documentation. The Unarchiver can be used on the Mac to join the files and uncompress them.
Note: Please test and give us your feedback in the comments. Make sure not to 'Update' the Firmware at any step, only use the 'Restore' feature.