Dempsey's future on 'Grey's Anatomy' a question?

Yes, "Grey's Anatomy" without Patrick Dempsey would be like "Glee" without the entire cast. How could it go on? (Of course, it could -- I'm just exaggerating for dramatic effect).

Dempsey was on  "Late Show with David Letterman" Wednesday and addressed his future. He said that indeed he has no contract, and indeed, he could end up leaving. But he also told an obviously worried Dave not "to worry."