I just took a 1.1.2 iPhone and decided to downgrade it to 1.1.1.
From my testing now 1.0.2 downgrade is necessary.

Prerequisite:
- you need to have the firmware 1.1.1 ipsw file in your computer

NOTE: This guide does not guarantee it will work 100%. Also errors 1015 is expected at the end of the process. Continue reading to understand.

1. Download the 1.1.1 ipsw firmware file from Apple. If the downloaded file has the extension ".zip", please remove it and modify it to end up with a filename ending in _Restore.ipsw

2. Reboot your iPhone holding the top (power) and home buttons BUT release the top button 10 seconds into it (right after the screen goes dark) and continue to hold the home button until iTunes detects the phone in recovery mode. The iPhone screen will appear to be off, but start iTunes if not started yet .

3. Restore your iPhone by pressing and holding the ‘Shift’ key on windows or ‘option’ key on Mac, then click ‘restore’ to select the 1.1.1 firmware file you downloaded earlier. The restore should go through and errors at the end with error 1015. However you will notice that the iphone is in DFU mode with the connect to itunes screen from 1.1.1.

In order to kick the phone out of that mode I just had to launch iNdependence version 1.2.5 and wait a minute or so.

Now you are ready to follow the easy steps at the following LINK in order to reactivate the phone:
http://iphoneunlocked.mostofmymac.com/Unlocking_Instructions.html

PS: And for those who wonder about IPSF, I tried to run it on my downgraded iPhone and it could not unlock the phone. It errors with "Unable to download firmware". We should expect and update from IPSF shortly!

I hope this helps.

TheMacThinker
http://iphoneunlocked.mostofmymac.com

Hey Man,

Nice Guide, but i heard that even after downgrading from 1.1.2 to 1.1.1 that the tiff exploit is still not working, can anyone confirm??

Tuaw reported that DFU mode has been turned off in 1.1.2?? Can you confirm that your step 2 works on a 1.1.2 phone?

so can somebody confirm this?
also are we talking about OTB or upgraded to 1.2.1

you need to use the old 1.1.1 firmware (do not download it from Apple's website)

anywhere we can find this file?

MostOfMymac.com!!!

How would I continue after step 3 if I have Window? Does I Brick do any goods?

thanks.

I did this in iBrickr and it downgraded just fine

It has worked for me and that is the reason why I posted this guide.

Yes like I said I managed to do this and reactivate the phone through the tiff exploit and that is the reason why I thought that this could help others