Sean Penn will miss the glittering premiere of “Fair Game” — in which he plays the husband of CIA agent Valerie Plame (Naomi Watts) — Thursday at the Cannes Film Festival. He’d rather be in Washington, DC, where he’ll testify at a Senate committee hearing on the mess in Haiti, his publicist said. But Saturday night, he had other important things on his mind and was spotted partying at South Beach club SET, hanging at a VIP table and “chatting up a long-legged blond model” until the early hours, a spy told us.