I've researched the Dean's Cert requirements for the 2008 admissions cycle for the entirety of the T-14, sans HYS, and only Cornell requires a Dean's Cert for an application to be considered complete.
Columbia and UPenn require Dean's Certs prior to matriculation, but do not require them for an application to be considered complete.
However, several schools' admissions staffs have mentioned that they do require Dean's Certs for students who have been thru academic probation and the like. Since I never had these experiences, I did not note which schools had this requirement, so if you have had these experiences, double-check on your own.
HTH