This article by George Monbiot in The Guardian says it so well, I'll add only this observation. Since I've become a somewhat recognizeable antiwar person locally (my ubiquitous sign gives me away), I've been accosted by these nutcases many times, most recently at an art gallery opening. The fellow sidled up to me and hissed into my left ear, "You know about the third tower, right?" When I said, "Uh?" he went on. And on and on, whispering "facts" (a typical one: "Jet fuel is basically kerosene, and can't possibly burn hot enough to melt steel girders") into my ear, moving to my left as I tried to turn toward him. I finally faced him and said, "Interesting."
Unlike Monbiot, I've learned not to engage these oddballs. I don't humor them, but I don't argue with them either. I say, simply, "Interesting," and move on. Maybe, I reason, they'll thereby lose interest and restrict their rants to each other.
I do have this query, however: How crazy do you have to be--how deeply paranoid about government, how downright delusional about the reality of events and human behavior--must one be to foster these conspiracy theories? I mean, I can understand beliefs in burning bushes (no pun intended), immaculate conception, even Roswell (although that's a stretch for me). These have some spiritual basis or factual possibility. But a mass murder and a mass coverup like 9/11? All I've got to say is, There are crazies on all sides of us--and each of them has one vote. So, guys and gals, vote early and often.