... and Bruce Willis like, the sky marshall springs from his economy seat and simultaneously manages to 'neutralise' all of a determined team or terrorists single handedly? At least that's how it works in the movies ... but in reality, would a single guard be enough on say a 747? Can't really visualise it myself ...
Personally I subscribe to the 'don't let the blighters get on the flight' school of thought. Having managed to walk through the security screens unchallenged at LHR with my mobile phone in my shirt pocket, and upon pointing this out to the security personnel there that said 'yeah that happens sometimes', it seems obvious that the thing to do before taking this crazy step is to beef up the existing arrangements so that they are at least adequate.