Well, don't forget the kid who won his school's Science Fair when he got 43 of 50 students to sign a petition to ban dihydrogen monoxide. See snopes
Having supported myself in the past by working the phone for some market survey companies, my two cents is that there are reputable survey designers and unscrupulous ones. The reputable ones know all the problems raised in this thread but know how to design their surveys as objectively as possible and present the answers honestly and as a result can charge enormous amounts for their work.
In Britain even if you are just working the phone asking people what they last saw at the cinema or if they have a B&Q store card, you have to be a member of a professional body. Sure it's easy to join but it does ensure some awareness of ethical standards.
As with any other field there are charlatans out there peddling unreliable nonsense.
Top tip if you live in Britain and don't want to answer a survey. Just say "I'm a member of the British Association Of Market Researchers and you'll know what that means." To which the reply should be, "Ah of course, I can't survey you."
