i found some stat for waitlist 2007-2008
http://www.cmu.edu/ira/CDS/cds_2007_...0Admission.pdf
it says on there: 2007 -2008
number of qualified applicants offered a place on waitlist : 2898
number accepting a place on waiting list : 290
number of waitlisted students admitted : 82
from the data given, it seems like 10% of waitlisted ppl remained on the waitlist. about 28% got off the waitlist eventually. i am not sure if all 290 people were on the priority waitlist.