GREY'S ANATOMY - &quot;Piece of My Heart&quot; - 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 &quot;Grey's Anatomy,&quot; THURSDAY, MAY 1 (9:00-10:01 p.m., ET) on the ABC Television Network.  (ABC/SCOTT GARFIELD)KATE WALSH, PATRICK DEMPSEY