schools have varied the way they operate waitlists from year to year. just because there was a priority waitlist last year doesn't mean there is this year. for example, ND has used a three tiered system in the past, but this year is grouping candidates into just two groups.
i had read somewhere that someone received what appeared to be a thicker envelope. mine was the same way, and i actually thought it might be an acceptance envelope when i saw it in the mail, but it was just a waitlist with thicker than normal paper and there was no mention of a priority list. i haven't seen any thread where someone has specifically said that they are actually on the priority waitlist, so if the school isn't admitting it, and everyone here has merely said they "heard" about it, then i'm assuming that for now there is only one waitlist.