Serena Williams -- already confirmed by the USTA
Chanda Rubin -- basically confirmed when I asked her about it last week
USTA National 18's winner -- automatic entry
Stevenson got snubbed last year and hasn't done much since, so I think that ship has sailed. Carly Gullickson has sort of gone backwards over the past year and may not even be the best prospect in her family.
A really good case could be made for Mattek, especially given how much younger she is than Washington, but they might think four wild cards in the past five years (without a win) is her limit. Interesting that the last spot could come down to a choice between those two, since they've been playing doubles together this summer. USTA's probably hoping for a few players to drop out so Bethanie can get in on her ranking.
One extra factor that I'm not sure about: the USTA recently extended its wild card exchange deal with Tennis Australia. Last time around it was only for men's singles (Mark Philippoussis into the '05 US Open; Amer Delic into the '06 Australian Open). Unfortunately, I can't find an article about it anywhere and I don't recall if the women are included now or not. If they are, you'd have to think Alicia Molik would be first in line.