On the men's side, the ATP rankings indicate that Team USA is most likely going to include John Isner(#10), Andy Roddick, (#32) Donald Young (#48), and Ryan Harrison (#52). #12 Mardy Fish has announced that he is not playing due to his recent medical situation with his heart.
There are also doubles teams (women's, men's and mixed) from each country. Obviously Liezel Huber and Lisa Raymond and Bob and Mike Bryan will be included, but the mixed doubles team has to come from the people who qualify for singles. That has lead to an awful lot of speculation about who will be playing with Serena: Isner, one of the Bryan brothers, or Roddick? Serena tried playing with lefty Bob Bryan in Paris and they lost in the first round. It will be interesting to see who she played the Wimbledon mixed doubles with. Obviously, she'll play the doubles with Venus.