um...
I believe that would fail, based on a concept in law known as ex post facto.
One cannot be charged with breaking a law prior to the law's existance. If you're seen walking down the street with a shirt on that says "PIRATES" and the next day they outlaw shirts that say "PIRATES"... you cannot be arrested for wearing the shirt the day before.
This concept carries to contracts and intellectual property, I believe. You cannot hold someone responsible for "violating your copyright" prior to your ownership of said copyright.
If AT&T were to reacquire UNIX code, they could levy charges... but I don't believe SCO has legal grounds.
Also, didn't AT&T actually TRY TO STOP THEM back in the 1980s and fail?