arwen15, WashU doesn't flat out waitlist "overqualified" candidates, but what they DO do is waitlist candidates who are obviously qualified but haven't shown much interest in actually attending the school. At our school, most of the HYPS...etc. bound students "throw in an application to WashU". Alot of them never visit the school and aren't really seriously interested, they just know it's a great school and that it's an easy application to complete since there is no required supplement. About half of our "cream of the crop" students who applied to WashU were accepted and half waitlisted. My guess is that the half who were accepted actually visited the school and had some real interest.
WashU wants kids who really want to be there, and the wonderful, positive atmosphere on campus is proof that their admissions approach works.