I was at Northwestern in the last week, and the dean said they would be sending out decisions next week for the people who interviewed "before." I don't know if that means "before" my interview or "before" February.
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."
On a side note, for those of you who have been offered an interview and are debating whether or not to go, I would say definitely go! I was so much more impressed with the school after being there and spending the day. The students were awesome and seemed so laid back! Best students I have talked to so far! I can't believe I'm actually considering paying so much to go there, but there was just something about it...