Atrios points out that Dennis Miller is something of a hypocrite in calling for the Left to stop comparing President Bush to Adolf Hitler. Personally, I can't stand the evocation of Hitler, both because it's almost always unfair and because on the rare occasion that an analogy to the rise of Naziism might not be totally out-of-line, the mere mention of such seems to be taken as permission to ignore it completely (or mock it derisively).
But I do have to note that the "How dare you call Bush a Nazi!" meme is a lot more hypocritical than just coming from the lips of Dennis Miller. As Atrios points out, it's not the Left that has used "Hitlery" as a pseudonym for Mrs. Clinton. It's also not the Left that injected the word "feminazi" into the American discourse. This rhetoric takes place on the far reaches of both sides of the aisle, but somehow you only ever hear about the far-Left calling Bush "Hitler" but never the same rhetoric from the Right, directed at the Left.
Just to take "Hitlery" for an example, a Google search of the Usenet database turns up over 2000 instances of "Hitlery" just in the last six months. And when you refine the general Google web-search for "Hitlery" to just pages updated in the last three months, over eight thousand hits result.
I would also suggest that, for the Right, the ultimate evil seems to actually not be the Nazis but the Communists or the Marxists or the Socialists, words which seem to be interchangeable in the right-wing lexicon. (After all, Reagan has to be the greatest hero in American history for something, right?) Somehow, I'd lay odds that characterization of prominent Democrats -- or Democrats in general -- as "Communists" is at least as common from the Right as comparison of Bush to Hitler is on the Left.
It's very tempting to use extreme rhetoric, but that doesn't mean that we should. And neither does it mean that we should delude ourselves into believing it only comes from the other side.
(Somewhat related to my point is the "cheapening" of our memory of the Nazis that Guy Gavriel Kay cites as a problem with the film The Patriot in this essay from a few years ago.)