The Republicans are having fun beating on John Kerry again. The stated reason for their anger is that Kerry implied only the lazy and uneducated dregs of society go into the military. The real reason for their focus on Kerry is that he was such a terrible, weak candidate last time around, they would love to run against him again, rather than whomever they are actually facing in their Congressional races this year.
I do think Kerry's remark betrayed a general liberal assumption that the army is made up mainly of uneducated southern rednecks and poor urban blacks. Its hard to get an exact demographic handle on this, because the army does not really ask how much money your parents make when they recruit you. However, they do ask you what zip code you are from, and this analysis seems to belie the common liberal assumption, showing the army is actually made up disproportionately of recruits from the richest rather than the poorest zip codes.