Many readers are reporting that — as suggested last night — Apple has fixed its issue with its activation server that the SAM unlock method relied on.
I've tested the tried and true unlock method that I posted yesterday, and indeed, it's not working for me either.
For those that did successfully use the SAM tool to unlock your iPhone before today, congratulations. Now we just hope that you are proactive enough to backup your activation ticket either manually, or via RedSn0w...