Downgrade iPhone 3G 3GS iOS 4 GM to OS 3.1.3 | iPhone

Downgrade iPhone 3G 3GS iOS 4 GM to OS 3.1.3 | iPhone: "and now you want to go back to OS 3.1.3 or lower for whatever reason, we created a step by step guide for iPhone 3G and 3GS running on iOS 4 GM. iOS 4 is still in GM and you needn’t to edit your host files for iPhone 3GS to bypass Apple signatures."