There was a story over the weekend in the London Telegraph about the European Union funding a research project at the University of Reading through its Security of Aircraft in the Future European Environment (SAFEE) initiative which is aimed at onboard threat detection, i.e., detecting suspicious behavior on passenger planes using "a combination of cameras, microphones, explosive sniffers and a sophisticated computer system" to analyze all the information. The idea is to detect potential suspicious passenger activity which would then be relayed by the system to the pilot, who could then decide what to do next.
Professor Ferryman is quoted in the story as saying, "What we are doing is extending technology already used at airports and railway stations and placing it on an aircraft."
The Telegraph goes on to say that the surveillance system would be made up of an integrated set of cameras and microphones that are spread throughout an aircraft, looking and listening for abnormal passenger behavior and or conversation. Bomb sniffers would also be placed in aircraft lavatories to detect whether bombs were being assembled.
The UK Department for Transport is quoted as saying that it has no plans to tell airlines to install such a system, but I bet it is only a matter of time.
Whether such a system would work well or not is open to debate, given current CCTV experience.
I would also be curious to see how well such a system handles hour after hour of screaming babies on a transatlantic flight (personal experience tells me that passengers nearby would increasingly appear edgy, annoyed and or angry and probably make semi-threatening comments under their breath about the second hour into the flight), or how the system would deal with over-hearing snippets of dialogue from someone watching an action movie on their personal player or through the aircraft's inflight entertainment system.