GREY'S ANATOMY - "Piece of My Heart" - Addison returns to Seattle Grace to perform an operation and is taken aback by all of the changes in her old co-workers' lives. Meanwhile Meredith and Derek's clinical trial has its first patient, and Rebecca/Ava returns with shocking news for Alex, on "Grey's Anatomy," THURSDAY, MAY 1 (9:00-10:01 p.m., ET) on the ABC Television Network. (ABC/SCOTT GARFIELD) JASON O'MARA, CLEA DUVALL, PATRICK DEMPSEY, T.R. KNIGHT