Finally some signs of life from the admissions committee! I got an email an hour ago asking me to confirm that I'd like to remain on the waitlist (bear in mind I applied R1). The email stated that they have not made any offers of admission from the waitlist yet, but that they have released "many" applicants from the waitlist (i.e. rejected), so apparently it is a good sign to still be in the running.
They asked to confirm interest in remaining on the waitlist by Monday 4/28. Sounds like they want to get a sense of R2 yield, and review R3 before making any decisions from the waitlist, so I'd guess early to mid-May would be best case for a final decision. The wait continues...