Get SHSH from iTunes backup?

Is there a way I can get my old SHSH from an iTunes backup? I've got a week 24 3GS running jailed iOS 4 right now.

Before someone asks, "OMG why you do that?!?" The reason this happened is I previously had a jailbroken iOS 4 3GS, but when I tried to install a fix for notifications in Cydia, I got stuck in a reboot loop and I had to restore my phone, and the only firmware that would work was the jailed iOS 4. I didn't know anything about SHSH files until after the restore (I don't keep up with this stuff.)

So my buddy told me he thinks there is a way to get an SHSH to downgrade the phone from an old iTunes backup, but he didn't know how to do it, and I can't find it with Google. Is he correct or mistaken?

I have the same problem. I had jailbrocken 3Gs with 3.1.2(+unlock). I've updated to 4.0 (without sn0wbreeze), cause I thought that there is jailbreak for 3Gs(like redsn0w...), after update I find out I was wrong :)
Now, my problem is, that I did not save .shsh file for 3.1.2 and now cant downgrade for example to 3.1.2 back, then jailbreak&unlock it, so that it will be possible to use my iPhone(now it is not possible because of SIMLock), I tried everything and come to conclusion that either I had to wait till new jailbreaking tool will released or I need .shsh file.
Now, my question is it possible to create/generate .shsh file from iTunes Backup, witch I did before updating?
I would be very grateful if someone can answer my question. Thanks.

For 3GS, If u did not save your shsh blobs ya cannot downgrade, so u will have to wait for comet to release jb tools for ios4.

Shsh blob are unique for each and every phone i.e. U cannot use someone else's shsh.

Guys, please search before posting. This has been talked to death.

If you didn't get your 3.1.2 or 3.1.3 SHSH, there is no way to do it now. You cannot fake it, you cannot sneak around it, you can't create it with an iTunes backup, you can't get it from someone you know's phone who had one, you can't use your friends, you can't edit the one you have for 4.0 or 4.0.1, you can't find a new way no one has thought of (or at least, we'd be surprised) -- it's done. You didn't get them. C'est la vie.

Get your 4.0 SHSH and 4.0.1 SHSH so you won't be in this boat in the future.

Closing thread.