Uh, the publicly-edited, well-sourced, and very thorough Sourcewatch wiki article isn't even in the same galaxy as "ad hominem".
Personally, I think it makes Cato look good. I don't always agree with Cato or what they do, but they are at the very least consistent in their principles and don't misrepresent the fact that they explicitly represent corporate interests, both things that are incredibly unusual among D.C. think tanks.
An ad hominem (Latin: "to the man"), also known as argumentum ad hominem, is an attempt to link the validity of a premise to a characteristic or belief of the person advocating the premise.
The suggestion was that something from cato was bad because its supporters were bad, which is ad hominem. The accuracy of the list doesn't change that.
see also http://www.prwatch.org/pdfs/2008_990_Disclosure.PDF .
What makes you think that those folks are on the up-and-up?
Ad hominem doesn't just go one way....