Credit cards are also required to hold a reservation on major Holidays. Several posters have reported that they needed to provide a credit card for their Thanksgiving, Christmas Eve and Christmas Day reservations. There is a charge if you fail to show up on those days, but no penalty if you cancel 48 hours ahead of time.
I believe there are also a few other situations where you are either charged ahead of time or need to provide a credit card to hold the reservation (i.e., dinner shows like Hoop-De-Doo & most dinner packages like CP or Fantasmic and IIRC, the Chef's Table at V&A's).
However, most PS arrangements (other than breakfast at CRT) can be made without a credit card.