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).
Looks like Apple’s currently fixing their activation servers. But the good news is that previous tickets gotten via SAM trick still work.
MuscleNerd has mentioned that the SAM unlock method will be a hit and miss until Apple rolls out the changes to block it across all the activation servers so you might still have a chance to unlock your iPhone using SAM. So check out this step-by-step guide to see if it works for you.
It is a pity that carriers in the US don’t unlock iPhones after users have signed for a 2-year contract (like carriers in some countries) as it can be very useful while traveling abroad. Hopefully, AT&T unlocking off-contract iPhones is a step in that direction.