As far as I know, using an eligible Platinum or Centurion card has always been a documented requirement of FHR. Of course, individual properties have been willing to accept different forms of payment as described in this thread.

Edit: I see Mia beat me to the punch and pre-emptively proved me wrong.

Edit 2: The same wording exists on the Canadian site:

1. FINE HOTELS & RESORTS – To receive the FINE HOTELS & RESORTS program (FHR) benefit package and rates, reservation must be booked with a Canadian American Express Card in the Platinum Cardmember’s name in FHR negotiated rates and room categories through Platinum Card Travel Service. Payment must be made with an American Express Card in the Platinum Cardmember’s name. Platinum Cardmember must travel on the itinerary booked under FHR reservations. Limit of one (1) benefit package per room per stay. Benefit package can include Noon Check-In, Room Upgrade, Daily breakfast for two people, 4:00 pm late check-out, plus an additional special benefit. Noon check-in and room upgrade based on availability and provided at check in. Certain room categories are not eligible for room upgrades. Call Platinum Card Travel Service for details. Breakfast varies by property but will be at minimum a continental breakfast. Additional special benefit varies by property, is subject to change, and cannot be redeemed for cash. FHR cannot be combined with other offers unless indicated. Benefit package expires at checkout. Platinum Cardmembers may book up to a maximum of three (3) rooms under the FHR Program. In the event the Platinum Cardmember cancels the reservation(s), FHR rates and benefit package will no longer apply to any remaining reservation(s). Back-to-back stays within a 24-hour period at the same FHR property are considered one stay. Participating providers and benefits are subject to change.