Joel Grey has been cast to play the role of a very important teacher from Izzie's past. He'll be making his way to Seattle Grace because he'll need her help. According to Shondra Rhimes, "He was the teacher who convinced Izzie (Katherine Heigl) she was more than a girl from the trailer park."
Sounds like this guest star should make for an interesting episode and also further proof that Izzie will be back after all.
Also, Rhimes mentioned the possibility of having Sharon Lawrence back as Izzie's mom, Robbie Stevens.
Source: TV Guide Magazine