Somebody else will be able to answer more certainly, but I know I was asked for my reservation number when I called to make dining reservations.
edited to correct my thoughts:
After reading Mick's post I went to Allears.net and confirmed what Mick said:
"Who can make Priority Seatings?
It used to be only guests staying at WDW or Downtown Disney resorts and hotels could make PS. Now, anybody can."