I'm in the PhD program at Columbia...from my limited experience, I would say there's a pretty open rapport between doctoral and M.A. students despite the fact that the dept. seems to encourage the formation of separate tribes...the only caveat would be that a few professors tend to prefer doctoral students for seminars, however I have taken a few with near 50-50 make-up of PhD/MA students.
Feel free to ask any more specific questions, look forward to seeing you here.