Even though a waitlist isn't what you were looking for, congratulations are in order because you're still in the running. I don't know much more than you, but I did read somewhere that ~50% of those that interview are accepted, meaning that there is a lot of movement on the waitlist. Oh, I also read that the Adcoms like to see LoI's and update letters. I would probably take all of this with a grain of salt, but LoI's and updates can never hurt your chances, right?
Anyways, good luck. By the way, did they ever imply that they are at capacity for straight acceptances and that they are pretty much only offering waitlist spots? My interview is like the third to last date they have and I'm a little worried since it's my only interview thus far.
