A simple answer: Apple doesn't allow you to do so.
There is only one solution to do this: If you backup your shsh certificates (specific for every phone) then you can backup to the version you backed up. Right now you can backup version 4.0.1. Even if you had 3.1.2 installed, you can only back up version 4.0.1 right now, because Apple issues only certificates for 4.0.1. Maybe you think going "back" to firmware 4.0.1 won't help you now, you might be right. But you may think different in a year or so.
How to backup: Use the software TinyUmbrella from here:
The Firmware Umbrella
Current version of TinyUmbrella is 4.01.01. Read also the README file carefully. There is much more information in there.
(Press Thanks if this helped.)