Although the vast majority of forum posters are reporting that their current handsets have never been jailbroken (it is an official Apple Forum after all) digging deeper into the threads it turns out that a large proportion of the users experiencing difficulties have, at some time in the past, either jailbroken another device or allowed their computers to be used for such nefarious purposes by a third party.
In nearly all of the cases, iTunes gets most of the way through the install process but eventually returns an error 1013 and then throws the iPhone into an infinite restore loop which can only be stopped by doing a hard reboot - hold down home button and power button for about 15 seconds or more until the Apple logo appears.
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