Visiting PokerStars.com, AbsolutePoker.com, or FullTiltPoker.com today will present you with the above warning. It seems the FBI has seized the Internet gambling sites' domains due to the owners being charged with violating U.S. anti-Internet gambling laws. This occasionally happens to popular Internet institutions, as it did with popular sports streaming sites back in February, though the sports streaming sites didn't have commercials air between nationally televised broadcasts, which is something a couple of these poker sites did, which made them seem legal enough. We haven't been able to find information regarding users' accounts as of this time.
UPDATE: If you visit any of the sites in question and notice that they are still up and running, it's simply a symptom of DNS propagation, which means whenever the change hits your neck of the Internet, the wonderfully terrifying FBI warning will replace your beloved poker site.