Here's the awesome news from those great guys at Entertainment Weekly. [link]

The redhead is back at Seattle Grace! ABC has confirmed that Kate Walsh, whose
freshman show Private Practice is on hiatus until fall, will return to Grey's
Anatomy as Dr. Addison Montgomery for one episode this spring. Executive
Producer Shonda Rhimes is still fleshing out the script, so there's no word yet
on when Walsh's episode will air. ABC will air five originals of Grey's
beginning April 24.