Has the dev-team releases a version of redsn0w that allows this re-restore function yet?

Please share all steps if someone got the way to downgrade with iOS 5.1.1...

The latest version of redsn0w supports managing a downgrade depending on whether you have valid SHSH (not all SHSH are qualified/valid these days). According to the blog post, you should be able to downgrade so long as you have a 4.x SHSH and a 5.x SHSH. You can't do it solely with a 5.x SHSH unfortunately.

I attempted this after upgrading my iPad to 6.0.1 and had no luck -- in my case I believe it's because I'm using the CDMA iPad2 and the restores fail when validating the SHSH files (even though based on my upgrade history, they should be valid).

There are a few tutorials around to manage it, but my understanding is:

1) Download redsn0w, an original .IPSW for a 4.x firmware you have an SHSH For, and an original .IPSW for a 5.x firmware you have SHSH for.
2) Use redsn0w to complete the restore. Point it at the 5.x IPSW, it will then ask for a 4.x IPSW and then ask for your SHSH. If your SHSH are on file with Cydia, then select remote SHSH. If you've got them as files locally, select local SHSH.
3) It should then reboot the phone and complete the restore process.

In my case, I get to the point where it asks for SHSH and it just says the SHSH don't seem valid, even though redsn0w itself will validate the SHSH files locally and remotely. The general guess in the support comments forum is that it just doesn't support the CDMA iPad2 yet. Admittedly, they did very clearly state that the iPad2,3 is the one that got the least testing.

I kind of wish I could ditch this iPad for one of the newer non-3g/non-CDMA ones, but I digress.

Just download redsn0w, you will get instruction also with it...

This is just to help out with more info....Please take a look at iClarified TUTORIALS/ iPhone / Jailbreak

iClarified - Tutorials - iPhone

then search down the list for your iPhone.... ie: i have iPhone 4 so i would search for How to downgrade your iPhone 4 from iOS 6.x to iOS 5.x for WINDOWS
iClarified - iPhone - How to Downgrade Your iPhone 4 From iOS 6.x to iOS 5.x (Windows)

thanks