Is Obama the "biggest celebrity in the world" as McCain claims?

A new John McCain commercial puts photos of Paris Hilton and Britney Spears next to Barack Obama and claims, "He's the biggest celebrity in the world, but is he ready to lead?" 

Tommy Vietor, spokeman for Obama, said in response: "On a day when major news organizations across the country are taking Senator McCain to task for a steady stream of false, negative attacks, his campaign has launched yet another. Or, as some might say, 'Oops! He did it again.'"