I think I figured it out. If your interview date is for march 2nd, I think you still have a shot at an outright acceptance, (the "few reaming seats" email a few of us including me got). If your interview date is for march 23rd, you're interviewing for a wait list spot. Is that consistent? Can anyone back me up...
This sounds right. I interviewed March 25 for a spot on the waitlist only. I was allowed to choose between March 25 and April 8, so I assume these 2 dates were the designated "waitlist only" days, they might have corresponding days for this year.