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