I was at Northwestern in the last two weeks, and the dean said they would be sending out decisions this week for the people who interviewed "before." I don't know if that means "before" my interview or "before" February or what.
But, she said that they will have all decisions made by the end of March. Actually she said, "if you don't have a piece of paper in your hand on the last day in March, go ahead and call us."
So no exact dates, but this week and then before the end of March.