Insults ≠ ad hominem fallacy

Hate to go all logic-nerd on you guys (well, actually I don't, but whatever).

Anyway, I recently ran across a misuse of the term "ad hominem attack," and I thought it would be nice to broadcast how the term ought to be used.

The argumentum ad hominem is an informal fallacy. It goes something like this.
A: "Spacesocks says that in a true ad hominem, one attempts to undermine another person's argument by attacking the character of the person making the argument, rather than by addressing the substance of the argument."
B: "That can't be true, because Spacesocks is a liberal elitist. She wants the government in charge of our health care! Clearly, you can't believe a word she says."

It's true that ad hominem arguments almost universally involve insults, but not every insult is fallacious, even in a rhetorical context:
A: "Michele Bachmann thinks we should investigate members of Congress to identify the ones who hold anti-American views."
B: "Michele Bachmann is completely nuts."

What's the difference, you may ask?

In an ad hominem argument, one uses one's judgments about the arguer as a basis for condemning a position

In a non-fallacious insult, one uses judgments about the validity of a position as a basis for condemning the arguer.