Yes, follow the posts I made above.
If you dont have your SHSH's you wont be able to restore to anything but 4.0
Or wait a few more days and you can update your phone, JB and unlock to the latest 4.0 firmaware.
Juiced you are the man, and I'm in the same boat as the OP so let me just double check:
-If run the backup through iTunes will I get all my crap back when I get my new iPhone 4? Phone numbers, etc...
-Cydia tells me my device has SHSHs on file for 3.1.3
-Do I need to download the 3.1.2 IPSW you posted? Or is there a 3.1.3 somewhere? I guess it doesn't matter right?
-Then edit the host file
-DFU Mode
-Hold shift and click restore to the downloaded IPSW
-Then run blackra1n to jailbrake and unlock