They had a thing on that "Reality TV Secrets" show about a guy who got stopped at the airport and it turned out to be Candid Camera. He not only didn't sign the release, he sued.
http://www.courttv.com/trials/news/0...amera_ctv.html
As far as I know, the people in the background need releases if their faces are clear enough to be distinguishable.
The old Candid Camera had silly things - like maybe someone dressed as a nun handing out condoms - but the shows now are getting more and more "extreme" and physical. If they keep it up there are bound to be more lawsuits.