This is just in my circle of friends and family, who run the gamut from pretty conservative to pretty liberal, but I don't hear anything like "Oh ****, we've got ourselves into another Vietnam" regarding either Iraq or Afganistan. Most are disturbed by the lack of WMD evidence, and most of them who supported the war are starting to question whether it was necessary or in our best interests. That may have an effect on the election, but as for a perception that this is becoming anything like vietnam -- well, its just a long ways off still.
Hell, I was very much against the war, and I think we've got to stay over there till we get a government in place that can stand on its own two feet. The absolute worse possible outcome of this would be for Iraq to become a radical, fundamentalist muslim theocracy that funnels its considerable resources into terrorist causes. If just left to chance, it is very possible that this would happen, and the new Iraq would be much, much more of a threat to the U.S. than the old Iraq ever was.
Anyway, I think it would take something like the tet offensive to really shake up the American public. Several hundred U.S. troops getting killed in a coordinated attack or something. Then you might see some real wavering in terms of people thinking we should stick it out in these countries.