Adding to what Josh says, presumeably Novak has appeared in front of the Grand Jury. Actually his latest column with references to "following [his] attorney's advice" and the fact that he was the first original ouster of Valerie Plame, there should no longer be presumptions. Novak appeared in front of the grand jury.
Now given that fact, don't you think that a man as intelligent as Prosecutor Fitzgerald after questioning Novak and recieving the answer "I got it from the Who's Who book" would have pretty much quit his investigation right there and then? I mean really, Who's Who? If it would have been that simple why the hell would this goose chase still be on, seems to me if it were that easy to dismiss, a certain prosecutor would not have let this go on for so long.
Josh is right, this isn't simply about where the name came from, but the motive to use the name, the choice of the word operative, and of course, where he got all this info. However, as Josh points out, it is extremely doubtful given the history and Novak's attorney prescribed silence, that he got it from a public source such as the Who's Who in America guide.