Today's Washington Post seems to imply that one of the leakers was Scooter Libby, Cheney's chief of staff. It looks like one of those deals where a source (in this case a Time reporter) says 'I'm not going to name names, but feel free to mention the following clues in your article ...." Here's what the Post said:
[Another journalist yesterday confirmed receiving a call from an administration official providing the same information about Wilson's wife before the Novak column appeared on July 14 in The Post and other newspapers.
[The journalist, who asked not to be identified because of possible legal ramifications, said that the information was provided as part of an effort to discredit Wilson, but that the CIA information was not treated as especially sensitive. "The official I spoke with thought this was a part of Wilson's story that wasn't known and cast doubt on his whole mission," the person said, declining to identify the official he spoke with. "They thought Wilson was having a good ride and this was part of Wilson's story."
[In addition to Novak's column, an administration official told The Washington Post on Saturday that two White House officials leaked the information to several journalists in an effort to discredit Wilson.
[An article that appeared on the Time magazine Web site the same week Novak's column was published said that "some government officials have noted to Time in interviews . . . that Wilson's wife, Valerie Plame, is a CIA official who monitors the proliferation of weapons of mass destruction." The same article quoted from an interview with I. Lewis Libby, Vice President Cheney's chief of staff, saying that Cheney did not know about Wilson's mission "until this year when it became public in the last month or so."]