iPhone restore error "This device isn't eligible for the requested build" - tried everything..? I restored my iPhone 4s yesterday on my computer and it went through with no problems.  

Today when my iPhone crashed and I tried to restore it again - I keep getting the message "This device isn't eligible for the requested build".
Nothing has changed from yesterday - so I don't know why it was OK then but not now...

I've worked through the apple suggestions - no problems with the hosts file, no third party security software, firewall off, etc  I've even tried it on another computer and I'm still getting the same message. It fails after the software is extracted (i.e. at the end of the first stage after I press restore) and it tries to verify with Apple...

Last bit of the restore log:
[11:33:28.4130] failed to personalize the restore bundle: This device isn't eligible for the requested build.
[11:33:28.4136] AMRAuthInstallDeletePersonalizedBundle
[11:33:28.4313] Restore completed, status:3194
[11:33:28.4313] Failure Description:
[11:33:28.4313] Depth:0 Error:Failed to copy auth install options in DFU mode.


Could the fault be on the Apple server side rather than mine or are there other things I should be trying?
Your problems are entirely on Apple's end. The message indicates that Apple no longer signing that version of software.

Yesterday, the code signing servers would sign the iOS 6 package you had downloaded on your Mac and let the device install that version of the OS.

Today, Apple is no longer signing that version of the software for that device.

I don't know how to say this other than Apple is forcing your hand to upgrade whether you are ready or not.

Had you used a jailbreaking tool to save the signature, you would have been able to replay that signing activity and possibly trick iTunes into restoring iOS 6. I wish I had better news, but you may have to jump to iOS 7 unless someone invents a new exploit to jailbreak iOS 6 in the absence of Apple's signing servers being cooperative with iOS 6.