It's appropriate when taken out of context with two generic dictionary definitions...

ad hominem attacks are never appropriate

This was not ad hominem, please look it up.

An ad hominem is used within the confines of an argument. There is no argument here. He is calling them what they are. They stole, that is clear, the argument is air-tight (they have even apologized).

I wouldn't use this kind of language myself (not in any circumstance), but ad hominem it is not.

Correct. I used the words literally, I meant "personal attacks" instead, which frankly I don't think helped him make his point stronger.

"Ad Hominem" doesn't apply here.[1]

(DHH didn't say that using 37Signals assets directly was bad because they were fucking scumbags, just that what they did was bad and they were fucking scumbags as a result.)

[1] http://plover.net/~bonds/adhominem.html

That's not true at all. They are appropriate when you are actually trying to deflate the opposing side's character. For example, if someone has a history of making poor choices, it's likely that any further choice they make is also likely poor. That's why judges often take into consideration whether an offense is someone's first, or someone's third.

With that being said, both sides definitely should have reconsidered their PR strategy. Both being techies, it's reasonable to assume that they should be fully aware of how things like this propagate on the internet.

So when someone makes the same mistake 3 times, you call them a moron and that fixes it?

Did you bother to read my entire post? The first part was correcting his statement. The second part denounced the insult in this case.

Yes I did. In any case an ad hominem attack is a logical fallacy, and as such it is never appropriate.