Sharing my experience..
I'm using iPhone 4 OS 5.0.1 jailbr0k3n, I backup all my data n 1st of all I proceed to upgrade my iP4 to 5.1.1.
I tested both OTA (firmware update via device - wifi) & also via iTunes usb "update" & "restore" (I'm using latest iTunes, tested diff ports, paused anti virus *kaspersky*) but it come to no avail. Encounter countless different error from 2digits (if not mistaken eg: 10, 15) to 4digits 4xxx, bla-bla.. Follow all the troubleshootings provided by Apple Support but still failed!! Search Google for solutions.. tested few methods (below) but doesn't work for me :
1. TinyUmbrella - enter DFU & fix recovery > then restore again - failed
2. TinyUmbrella - shift restore 5.1.1 while running umbrella's tss - failed
3. Enter DFU mode then shift + restore iPhone3,1_5.1.1_9B206_Restore.ipsw - failed
I've spent all above methods for hours but I never give up!! Lastly I found an effective method (at least it worked for me) :
Download redsn0w 0.9.11b4
Download iPhone3,1_5.1.1_9B206_Restore.ipsw (via:
iClarified)
Open Redsn0w > go to Extra > create custom firmware via "Custom IPSW" > select iPhone3,1_5.1.1_9B206_Restore.ipsw > it will create NO_BB_iPhone3,1_5.1.1_9B206_Restore.ipsw..
Finally! shift + restore via iTunes & make sure select NO_BB_iPhone3,1_5.1.1_9B206_Restore.ipsw!!
DONE!!
Successfully updated to 5.1.1 > then jailbr3ak via Absinthe 2.0, success!!
HOWEVER I AM NOT RESPONSIBLE IF SOMETHING WHERE GOES WRONG HERE. But seems to have fixed the issue for me with no problems. DO IT AT YOUR OWN RISK!!!
P/s: iP4s can update 5.1.1 via itunes without any prob (tested 1 4s - no prob). Above method for iP4 which I tested.. If 4s facing this prob, can try this method at your own risk..

Hope my method helps.. sorry if this method has been posted before me as it's kinda-sorta impossible for me to read-thru all the replies...
This post has been edited by Mr Cola: May 28 2012, 10:58 PM