Information for the taxi voucher service for students who live in Cambridge (beyond Central, Inman or Porter Squares) or Somerville can be found at http://www.law.harvard.edu/current/student-services/transportation.html#TaxiEscort.
Just remember that you should call a little before you actually need to leave campus because we use a Boston-based company, so it can take a little time. Please keep DOS posted of any problems with the service, though, as they want to make sure no one is having to wait more than 20-30 minutes for the cab to arrive.