So, I finally nailed a 3.1 to 3.0 3G[S] downgrade using "on file" ECID SHSHs. Will have more complete instructions for doing it tomorrow.
After iH8sn0w, a new tweet informed us of the success in downgrading the iPhone 3GS firmware 3.1 to firmware 3.0-- so there is a possibility of jailbreaking the device. All devices that used the ECID grabber from the server via Cydia are safe!

Using these files will also be possible to make this process even using iTunes, thus making things much, much easier. Saurik states to issue the guide to downgrade the firmware tomorrow.