I've certainly only used it or seen it used as a generic, gender-neutral term. (And only in the phrase ad hominem.)
As Kosmonaut mentions, part of it is that it's in a different language, so any gender mismatch* is not going to be apparent to people unless they know some Latin. But there's also the fact that classically, in many languages the word for "man" had two meanings: (1) a masculine person, and (2) a member of mankind; a person (of unspecified gender). Modernly, the second meaning is sometimes seen as prejudiced against women, and thus people come up with all sorts of newfangled constructs to avoid that usage. Thus you get (hopefully**) deliberate plays on words like "ad feminam", which doesn't actually mean much of anything, but is understandable if taken in parallel with ad hominem.
* As JSBangs points out, homo is not actually the Latin word for a specifically masculine person — that would be vir — so there actually is no gender mismatch with using ad hominem when the subject of the attack is female.
** If the usage was not deliberately playful, but meant sincerely and literally, I would find it to be sad evidence of the decline of society.