I spoke with someone on the admissions committee and he stated that they would begin looking over WL candidates later this month and that the first round of decisions will be sent out early June. After the first round WL candidates will be accepted, rejected, or asked if they want to remain on the presumably shorter WL. I hear that this business of getting waitlisted onto another WL takes place all summer long. Officially they have a first, second and third 'Reserve Group'. The first is the one we are currently on and if you're asked after this round if you want to remain on the WL you say yes, you will then become a part of the second Reserve Group.
Hope this helps. Any idea how large the WL is??