I listen to an interview this morning of McCain, and, contrary to media hyperventilation, McCain certainly did not indicate that there was any chance he would be on Kerry's ticket. He said that he would "entertain" an inquiry from Kerry because they are very close friends, but that he could forsee no possible circumstances in which he would be Kerry's running mate. Furthermore, he said that he planned to campaign for Bush.
(I assume he didn't make a seperate statement elsewhere contradicting this, and that the media is commenting on the same interview I saw.)