The single actress, 40, ferociously flirted with Robert Pattinson -- who reconciled in early September with Kristen Stewart -- at the Gucci-sponsored L.A. County Museum of Art's Art + Film Gala on Oct. 27.
"She was pretty obvious," says one of three witnesses who watched Diaz come on to the Twilight star, 26. "Cam was seated next to Rob at dinner. She was touching his arm, doing her big Cameron laugh at everything he said and trying really hard. He was polite, but not having it."