If you have a new iPhone 3GS bootrom with firmware 3.1.2 installed on it now or you have SHSH for 3.1.2 saved in Cydia or on your computer then you will be able to restore to iOS 4.0 jailbroken using a new version of sn0wbreeze that will be available tomorrow. iH8sn0w is working to modify sn0wbreeze so that any iPhone 3GS owner with firmware 3.1.2 installed on the phone can update to jailbroken iOS 4.0. If you do not have firmware 3.1.2 installed in your phone and you do not have SHSH for firmware 3.1.2 then YOU CAN NOT use tomorrow's sn0wbreeze release.
You can't save SHSHs for firmware 3.1.2 now, you can't UNDER ANY CIRCUMSTANCES restore to firmware 3.1.2 if you do not have SHSH saved and there is no other method to return to 3.1.2 without SHSH. If your phone is in Recovery/Emergency Mode then you have NO way to find out what SHSHs you have available in Cydia for your phone because there is NO way to find out. The only thing you can do is follow the tutorial this and try to restore to firmware 3.1.2. If you receive error 3194, it means that you did not follow the tutorial properly and did not modify the hosts file correctly. If you receive an error saying that the phone is not eligible for downgrade, then you do NOT have SHSH for firmware 3.1.2.
Please read carefully what I wrote because I don't intend to repeat myself in the comments.
