My take on it is just that they search their own users first, then broaden their searches to include Kazaa and Grokster clients. Everytime I look at a download it starts off with maybe 2 users, then all of a sudden the # of users jumps to 7 or 8. Never gradually so i figure it's starting to search other clients rather than it's own pool . iMesh makes a download manager based on it's core business and could easily organize it's searches that way.
http://imesh.net/