Not quite. Northwest have a 'golden share' in Continental which up to now has blocked any merger by CO with one of NW's rivals - namely United. This share, which apparently has a nominal value of $100, is no longer effective in the event of NW merging with anyone else, which now leaves UA and CO free to get together.
An American/Alaska get-together has also been touted. Which would seem to leave US Airways out in the cold.