If they play Saturday, they are obligated to regard the game as the Opener, and today's tickets would be good for that game. They'd have to postpone the scheduled game, and perhaps play a doubleheader Sunday.
This is what I was wondering about. I have tickets on Saturday so selfishly I was kind of hoping they would play that game as the Opening Day game, but that wouldn't be fair to all the people who had tickets to tonight's game. So I assume it is the Saturday ticket-holders who will be rescheduled.
I just heard from my Nats ticket rep and she said an email should be coming out shortly, but it sounded like no firm plan has been decided yet.