Just to add to the ever growing pile:
Michigan resident, interviewed Feb 15th, Delayed Decision, Waitlisted Letter received today May 14th.
I will be declining my WL position for those who are really interested in knowing that sort of thing.
My previous experiences with Tufts: personally, while Tufts admission policies do kind of suck, as far as the perpetual "delayed decision, where do I actually stand, I never hear anything from you" sort of thing, they are by no means exclusive to Tufts. Quite a few places did exactly the same thing i.e. "we're not really sure, we'd like to see if we can interview anyone better, so we're going to put you 'on hold,' not a waitlist, not a rejection, basically no decision really, just wait 'till our people call your people."
In previous years, I had also been waitlisted at Tufts, and well, obviously, never got off the waitlist. In that year, I had interviewed in late April, so I was never put in the delayed decision category then. However, I do believe that prety much everyone (with a possible few exceptions) originally in the delayed decision pile gets waitlisted.
My experience with the Tufts waitlist--very poor. They say it's not ranked, but seriously, does anyone believe that? Now, they may not be numbering people from 1 to 500, but they must have certain groups of people who are "higher priority" than others. That's fine, it's makes complete logical sense, I just wish they'd be candid for once, and say so, rather than steering applicants' perspectives in another direction.
I do believe the waitlist at Tufts is quite substantial--they interview a high number of applicants, and pretty much put everyone in the DD category and then WL them--again I'm sure there are a few exceptions, but I'm speaking of the overwhelming majority. As to how much WL movement occurs, I can't say--I never got off the WL, but my cousin did (shortly before classes started), though she was a MA resident living in Boston at the time.
Anyways, those are my thoughts, ponder them, or not, as you will. To those waitlisted and hoping to get off and go to Tufts (or anywhere) I wish you best of luck!