Even as homeowners gleefully wire up their homes with inexpensive Web cams, even as employers put up closed-circuit TV and cities install surveillance equipment on everything from traffic intersections to school buses, a small group of skeptics is beginning to question the effects of all this technology. They ask: Will you trust your neighbor in the 21st century? Or in putting up a security camera — just to make sure — are we somehow pulling out an essential thread of the social fabric?