ABC announced and TV Guide reports that veteran actress Diahann Carroll will be guest starring on Grey's Anatomy during Season Three. But series creator Shonda Rhimes (in typical, secretive fashion) stopped short of saying who Carroll, 71, will be portraying.

Some guesses include the mother of Dr. Burke, or perhaps a new senior-level administrator at SGH. A counterpart and/or a rival of Dr. Webber, perhaps? We will have to wait and see. Carroll (below, left) has appeared in 49 feature films and is also a Tony Award-winning stage actress.

In other Grey's cast news, the scene-stealing Sara Ramirez (below, right), one of the show's notable guest stars, has been promoted to full-time cast member. The Insider would like to send our congratulations. We're already getting psyched for more awkward moments in the bathroom at Meredith's house whenever Ramirez's character, Dr. Callie Torres, spends the night!

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.