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MuscleNerd of the iPhone Dev team has tweeted that Apple has started fixing their activation servers, which means that it may not be possible to unlock your iPhone using the SAM trick.
The good news is that the backed up activation ticket seems to work according to MuscleNerd. So if you have managed to unlock your iPhone using SAM then please take a backup of the activation key either using iFile or Redsn0w using our step-by-step guide (here and here).