On that note, Liz, I found this interesting:
"Are illegal alien males required to register?
Yes. All immigrant alien males, other than those admitted on nonimmigrant visas, must register, whether or not they have a green card." (Emphasis added by me)
That's from the Selective Service website.
Which sounds like if your UK husband is between the ages of 18-26, he has to register if you move to the US!