We've got the scoop on the official date of the much anticipated musical episode, plus the very special guest who will return to Seattle Grace for it.

Kate Walsh will star in the episode, airing March 31, 2011. We know it's said to be a Callie-centric ep, so most people are speculating that Addison's presence means bad news bears for Callie's gestating baby McSteamy. But all miscarriage rumors have been squashed, so Callie will definitely carry the baby to term.

We're not discounting the possibility of Addison showing up to help Meredith and Derek in their baby-making adventures in some way (awkward though that may be considering she's Derek's ex wife). Maybe Dr. Montgomery has the cure for a hostile uterus. Dare we to hope that MerDer is already pregnant by then and the complication has to do with their pregnancy?

Stay tuned to Grey's Anatomy on March 31 to watch all the baby drama unfold...in song.

Source: TV Line