Received an email from [SuperBook]/[Sportsbook.com]. See below:
Due to the recent industry events we will no longer accept new signups from United States residents. Sportsbook.com, PlayersOnly.com and Superbook.com will cease accepting new U.S. players on May 1st, 2011.
It appears that current accounts will be unaffected. If you don't have an account at [SuperBook]/[Sportsbook.com], I'd consider getting one before the deadline. I'm guessing depositing and withdrawing will continue to get more difficult, however.