As many of you are now aware, Apple's cat and mouse game has ensured that most 3GS users cannot downgrade from 3.1 to 3.0 due to a code signing issue. Saurik however did open up a Cydia ECID storage facility just prior to the 3.1 upgared that 50,000 lucky 3.0 iPhone 3G users registered on, allowing them to ability to downgrade. As an exploit has been found in 3.1 it looks hopeful that this too will be jailbroken at some point soon. So to ensure you will be able to downgrade to in future you should have Cydia save your 3.1 ECID. Read Saurik's excellent article here for more details on this. It's a bit lengthy but highly informative and necessary reading for all 3GS owners.