I think the point is that we vigilantly distinguish "that was stupid" from "that was a stupid person". Judging someone as stupid means we've stopped thinking about possibilities. Derek wisely says there's always more to the story (reminded me of this: http://garrysub.posterous.com/what-does-it-feel-like-to-be-s...)
Labeling people is nearly always a substitute for thinking and empathy (both hard things).
Lest I forget, there is also ignorance-- willful cluelessness; can't be bothered to learn something; won't have a reasoned discussion. I do label these individuals "stupid." Don't get me wrong, it's not immediate prejudice, but it doesn't take much time talking to some people to learn that they are willful in their efforts to remain uninformed.
Labeling people is nearly always a substitute for thinking and empathy (both hard things).