Hey.. I have had this issue since yesterday afternoon.. Some people are saying to keep restoring again and again until it finally goes through.. I have done so like 10 times on my MAC and then I used my husband's PC and tried it 4 times.. No success.. I called apple support, but no help came from there at the end, and the only thing they were able to suggest to me is to sell me a new phone just like mine for a discounted price.. **** NO
Anyways after about 18h of search and stress I came across this link:
http://techglobex.blogspot.com/2012/06/downgrade-from-ios-6-beta-firmware-to.htm l
That will only make it possibe to go backward.. You will end up back with the IOS 5, but at least you will have a working phone and it will bring back all your contacts and everything else that you ever had backed up on icloud and on your computer. There might be descripencies with the Applications but again this is an issue easily solved, you can always download them back.
Two more things: One, you do not have to download the old version of the itunes, the newest version worked for me without any issues
When choosing to download the older IOS 5.1.1, you will need to figure out whether your carrier is GSM or CDMA but you can easily google this info online..
Well hope that will work for you like it did for me