By now, you've surely heard about Glee's Britney/Brittany episode -- a tribute to the music of the one and only Britney Spears. But what you may not have heard is that the pop icon herself will be making a guest appearance.Glee creator Ryan Murphy officially confirmed Britney's guest spot on Ryan Seacrest's radio show today. "Britney, I think 100 percent, is going to be on the episode," he says. He also went on to say that the really cool Britney stuff happens when the club members go under anesthesia at the dentist's (played by John Stamos) office. "They have fantasies of how they can be like her," Ryan reveals.