cl14, smearing theories (which can't be proven, even though some are probably basically correct, e.g. evolution) is allowed and isn't ad hominem, ever. Still, you are supposed on this board to back up any attack on a mainstream theory with facts, or admit that it is just a hunch and you have no serious arguments.
Furthermore, not all attacks on a person are ad hominems. If someone can show why he thinks that Einstein has conned us (e.g. a letter from him stating that "haha, I fooled them all"), then it isn't an ad hominem attack to do so. Similarly, if someone tells a blatant lie (like saying (I never claimed X or Y), then pointing that out isn't an ad hominem attack either, if you can show where and how the person lied.
On the other hand, saying "Einstein conned us" and then only trying to show that his theory is wrong, but not that he willingly or knowingly misled us, is indeed an ad hominem.
Another point: I don't think it is allowed to put a link to your own website, and then putting on that website "The BA stinks" (sorry BA, just giving an example). But linking to a third party website that contains ad hominems (say, Hoagland's website, or GLP) is something else. Although in the latter case it is best to warn people that the link can cause permanent brain damage, it can hardly be forbidden to link to that site, if it is pertinent to a discussion.
Of course, linking to shock sites, porn, or other unacceptable sites (for this board) is still forbidden, and has resulted in warnings yet. So basically I see no double standard.
To give one more example: if you would write :"Neil Armstrong is a nitwit", then you would use an ad hominem and have to face the repercussions. On the other hand, if you report on Bart Sibrel and quote him saying "Neil Armstrong is a nitwit", then you are not using an ad hominem (unless you continue with 'and I agree' of course).
The line between the acceptable and the unacceptable is of course not that clear as I try to show here, but I do think that most people know when they have gone too far and crossed it. And when in doubt, ask a moderator.