My take on civil defense is that it was a big joke, merely designed to make people "feel" better but not actually accomplish much else. All the "duck and cover" drills were setup to make people feel patriotic and secure in the notion that they were still the masters of their own fates and so forth. Nothing could have been further from the truth then or now.
Over at LewRockwell.com they had some articles about how the federal government had all kinds of plans for saving itself and imposing authoritarian rule (complete with the surviving citizens acting as slaves) in the event of a nuclear war. The point was that bureaucracies defend themselves while the citizens are hung out to dry. We are expendable. Which is obvious to anyone who has ever had their door kicked in by the BATF, but I digress.
Civil defense like the submarine watchers in the movie "1941" (y'know, the ferris wheel guys?) makes slightly more sense, I guess. Hollywood typically portrayed them as idiots though.
Troy is absolutely right--without the proper training and all kinds of equipment, you can't defend yourself against chemical and bio weapons. Luckily, their effectiveness is limited to their dispersion method and a crop duster with a 100gal. tank just isn't that big of a threat. They would need jets flying in formation to accomplish anything big. Think Agent Orange in Vietnam. This crop duster thing is just another scare tactic.
Poisoning a water supply or food supply would be much easier, more effective, and harder to defend against. NYC's water supply comes from big resevoirs upstate. I always hoped that upstate would someday revolt and cut off NYC's water. :-)