I had the same and was getting repeated failed restores with a variety of different error codes.
This is my fix:
This only worked using a different PC other than my normal sync, (glad I have 4 in the house). I dont know why this is the case, could be many issues from usb drivers to plugins but I dont care why it works, it just did.
I got a failed update from the itunes update, then failed restores for 5.1.1 through itunes.
I tried to restore from direct download file as well, many system reboots, service restarts, recovery mode phone reboots, etc.
I then attempted to restore 5.1 from the download at http://gdeluxe.com/ios-firmware-ipsw-download-links/ but that too failed.
So I was going to wait until my express lane apointment came up and figured I should charge up the phone so it was maxed out...but...frustration got the better of me.
I jumped on one of the other PC's which hasnt even been sync'd and hasnt had an itunes update either. It still has 10.5.2.11 version installed. Held down power + home button for reset and plugged the phone in
So I transfer over the install file to the new PC (file name I used is "iPhone4,1_5.1_9B179_Restore.ipsw" from the Download iOS 5.1.0 iPhone 4S link above) plugged in the phone, itunes starts up and asks to update the software, I deny it.
Then the popup for "Iphone in recovery mode...must be restored...yadda yadda" comes up, clicked OK.
Now shift clicked the restore button and, just as I was about to try 5.1.1 I figured I would just go with 5.1 version instead.
It installed without issue, my Iphone is now alive again and not just a drink coaster.
Took it back to my normal sync terminal, resotred my backup file and yay...no more 5.1.1 woes
Hope this helps others to repair their iCoasters.
Perhaps rolling itunes back to earlier version and installing the 5.1 can save some others from the 5 hours of iRage i suffered.
Repeated across other threads for those who are having trouble
