rhyme
Decisions are not rolling. The calls typically start 2-3 days prior to the actual decision deadline. Generally, with very few exceptions, if you don't hear by the day prior to the "official date" you are denied / waitlisted.
I can attest to this, as I was waitlisted twice. If you don't hear in the days before via phone, things aren't going to go your way.
Last year signaled a change from a WL standpoint. As far as I know, there was only one person that was accepted off of the waitlist. Rose mentioned a few times that they were oversubscribed in terms of class size. I would expect that this year, there will be far less WLers than in the past. Which means that if you really don't hear by the 18th, your admission chances are slim.
That being said, b-school has proven to be an enigma so far...