Shhh! It's a secret

There sure must be a lot of mistletoe on the set of "Gossip Girl."We hear that on-screen sweathearts Blake Lively and Penn Badgley — whose uptown/downtown relationship keeps the hit show sizzling — have quietly started dating in real life. "No one is really supposed to know that," laughs an insider. "I was told by a friend of the family."The new tryst has meant Lively disentangling herself from her former boyfriend, and Burbank High School classmate, actor/model Kelly Blatz. The co-stars were spotted shopping together Dec. 17 at ABC Carpet and Home on Broadway, looking every bit couple-like. Reps for the actors did not comment.