If the weather keeps ticket-holders home, the US Open would rather not show empty seats on TV, and may allow you free entry to the grounds and all the tennis venues, except for Arthur Ashe Stadium. Last year I got in for free to watch quarterfinal play for men's singles and women's doubles. It was great. See the photos I took. Just wandering around the USTA grounds is fun and there's always a chance to spot a celebrity or at least John McEnroe.
Of course, there's no guarantee the US Open will let you in for free this year, but just in case I'm watching the weather. Sometimes the news is announced on the radio (try 1010 WINS), and I'll make sure to pass it on through the Queens blog.