IHEART ATTACK: Performers and fans alike are headed to Las Vegas’ MGM Grand for tonight’s Clear Channel iHeartRadio Music Festival, which Ryan Seacrest will kick off at 7 p.m. (PT). @britneyspears tweeted: “Headed to Vegas for the #iHeartRadio Music Fest Sep 21 & 22. U can watch it live here: iheartradio.com.vegas Heard it's going to be insane..." @UsherRaymondIV, who is set to perform tonight, tweeted simply, "Viva Las Vegas!!" Rihanna, Lil Wayne, Green Day, Swedish House Mafia, No Doubt, Miranda Lambert, Jason Aldean, Bon Jovi and contest winners Megan and Liz are set to go tonight, while Taylor Swift, Aerosmith, Brad Paisley, Pitbull, deadmau5, Enrique Iglesias, Linkin Park, P!nk, Mary J. Blige and Calvin Harris will perform tomorrow night. The entire show will be streamed live in HD at www.iheart.yahoo.com. (9/21p)