This just in, from the Hollywood Reporter:

" Tim Daly is set to guest-star in the 'enhanced' two-hour episode of ABC's Grey's Anatomy.... According to the breakdown, Daly's character is described as 'handsome, sincere, like a McDreamy.'"

Paul Adelstein ( Prison Break's Agent Kellerman) also will guest in the episode, with an eye to costar on the potential Addison-centric spin-off toplined by Kate Walsh.