If we are to accurately describe "un-American," we must first describe the word "American," must we not? Here, I do believe that we refer that we use the word "American" as an adjective, that generally is used to describe someone patriotic, one who shares the same view as the speaker, and other such (idiocies?). Now, as to the meaning of "un-American", I'd say that many people would describe someone as "un-American" if they either 1) disagree with their views (and, in their opinion, would lead to something 'bad' happening to America) or 2) they are using it as slander against an opponent in a political race.
At any rate, I do believe that the term "un-American" is an opinion, and is only to be used when someone is losing real things to argue/debate about. Of course, it might depend on the situation, but the general consensus seems to be that calling someone un-American is to attempt draw attention away from a certain point, or some other political stunt, and is generally used by the one losing in a (certain) situation.
My two cents.