Thanks to Saurik for all of his hard work. Below is his explanation from saurik.com on how to downgrade your iPhone 3G S and also how to continue using his server for future Apple firmware updates.
Bypassing the Overlord
To this end, I have constructed a server that duplicates the functionality exposed by Apple’s signature server, except using “on file” results rather than live requests.
All we need, then, is to make iTunes use it. Luckily, most operating systems also have the ability to locally define bypasses on specific hostnames through a file called hosts. Using this, we can redirect requests to Apple’s signature server to Cydia.
So, open the file C:\Windows\System32\drivers\etc\hosts (Windows) or /etc/hosts (Mac OS X) and add the following entry to the bottom of the file.
74.208.105.171 gs.apple.com
Now, when iTunes thinks it is talking to Apple, it is talking to Cydia Read more…