since Joe Wilson's op/ed didn't appear until one month after the event mentioned in the OP.
Update: I found the reason for the button, and although it had an entirely different meaning at the time, it certainly was prophetic.
It seems from this article about the event, the 40th anniversary of Novak's column, that while he is an extremely partisan rightwing columnist, this event was surprisingly non-partisan.
Present were Democrats who in the past had been slammed by Novak in his columns. This article deals with why such individuals would want to associate with Novak. It seems there are several theories about that. One being that if you're on his good side, you might not be slammed as viciously.
Thus, the button which was meant to be humorous 'I'm a source of Novak's, not a target of his column' (hopefully, if I suck up to him enough). So 'target' in that sense, meant a target of Novak, not of the special prosecutor.
Novak's amazing longevity and range were apparent one night nearly two years ago at the Army and Navy Club, in Washington, when he marked the 40th anniversary of his column. Given his fiercely conservative views, it was a surprisingly bipartisan affair. Karl Rove was there, but so was Rahm Emanuel, formerly a key Clinton aide and soon to lead the Democratic Congressional Campaign Committee. Ken Mehlman, now head of the Republican National Committee, was there, but so was Bill Daley, who ran Al Gore's presidential campaign, and Bob Shrum, who would soon run John Kerry's. Long ago, Novak had trashed George McGovern, for whom Shrum once wrote speeches. And last year--and without acknowledging that his son was marketing it--Novak was pushing Unfit for Command: Swift Boat Veterans Speak Out Against John Kerry, the book that helped sink the Democratic candidate. Novak's attack on Kerry led comedian Jon Stewart to label him "a douche bag of liberty," and his failure to disclose his son's role in the book led the Washington Monthly to accuse him of operating in a "Cayman Islands-like ethics-free zone." But Shrum, like many liberals, still calls Novak his friend.
Another veteran Democratic operative, Mark Siegel, explained why. There's this Novak caricature on television, he said, but privately he's "a very kind, sweet, thoughtful guy." To Representative Barney Frank of Massachusetts, Novak's cross-party appeal instead reflects something putrid about political culture in the nation's capital. "It's the worst, most glaring example of people in Washington putting personal relations over their principles," he says. By liking Novak, he believes, people think they can prove how open-minded and civil they are. Indulging Novak's hatefulness, he says, is "one of the membership requirements of the Washington establishment."
Some glad-hand Novak out of fear or calculation more than affection. At the Army and Navy Club celebration, people wore buttons reading "I'm a source, not a target"--a nod to Novak's statement that for him people fall into one category or the other. Even those who find him crude feel elevated in his aura; hanging around him, appearing in his column, are signs you've arrived, even when he whacks you. Always, though, there is the debate over what is at the man's core. "Underneath the asshole is a nice guy, but underneath the nice guy is another asshole," Michael Kinsley, a Crossfire veteran, has said.http://www.vanityfair.com/commentary/content/printables/050321roco02?print=trueStill, there's some sort of Karma at work in that the picture now. He may have placed himself in the position of being at least a subject, if not a target, of the investigation by trying to cover for his boss, not reporting the leak to the proper authorities and lying to the FBI.