Kevin Drum links to a few more sources about Kerry's work with the CIA in Cambodia and has this to say:
Kerry certainly operated in the area of the Cambodian border in late 1968, Americans were definitely making border incursions at the time, and the CIA certainly had a lot of people in Vietnam in 1968. That doesn't mean Kerry's story is true, but it's certainly plausible.The serious evidence against Kerry seems to consist of two things. First, the Swift Vets group claims that "All the living commanders in Kerry’s chain of command . . . deny that Kerry was ever ordered to Cambodia." Second, both David Brinkley's biography of Kerry and Kerry's war journal mention only that he was near the Cambodian border on Christmas Eve, not across it. (Although the journal entry ends with a sarcastic message to his superiors: "Merry Christmas from the most inland Market Time unit" — at a minimum a reference to being right on top of the Cambodian border. Then: "You hope that they'll court marshal you or something because that would make sense" — possibly a reference to crossing the border.)
It doesn't really clear anything up, but it's (slightly) more information. I wonder if we'll ever know the truth on this one.