A lot of our trolls came in here yesterday whining that what the Joint Chiefs of Staff were doing by complaining about the Post's cartoon criticizing Rumsfeld wasn't censorship. Never mind that I hadn't claimed that it was, but anyway. It of course isn't strictly censorship, but any time a member of the government complains in this way, behind a government podium or on official letterhead, it does indeed get closer to official censorship. The point is to have a chilling effect. Had Rumsfeld or one of the members of the Joint Chiefs complained on personal stationary it would've been a bit better, though the dishonesty of the letter would've been still made it a bit awful.
The point is there's a big difference between someone like Bill O'Reilly saying "people should watch what they say" and Ari Fleischer saying it. Both are meant to intimidate, but one is an agent of the government and one is not. Both can have a chilling effect on speech, but only one has the official government approval on doing so. Censorship? Not quite. But creeping close to it.