I thought the only people briefed on this were on the Intelligence Committees. Is McCain on the committee? I didn't think he was.
Re: Intelligence committee -- McCain's not, but there's an interesting line-up of possible leakers, starting with the Vice-Chair, JR IV of WV (my opinion, of course -- and it looks like quite a few others are thinking along those same lines):
http://intelligence.senate.gov/members.htm