My previous diary entry "The Stripping of Freedom: A Careful Scan of TSA Security Procedures (video)" while popular as far as lambasting TSA abuse went, also advocated allowing guns on planes. This part was not so well received as the comments reflect. Let me elaborate on the subject.
I advocated "allowing selected passengers to carry guns on planes as a way to enhance security". This does not mean that everyone can bring any gun on board. The airlines, not the government, would regulate who can carry what. Not many passengers would opt to carry. Some gun owners may not have the right kinds of guns and therefore not qualify. Many lack training in using guns on a plane. The idea is not an armed mob but a few select people on the plane being armed.
The airlines should also be allowed to have their own armed security on board to replace the sky marshals. This security could be both uniformed and plainclothes. Of course, pilots that want to carry should be allowed to.
Given this kind of unknowable threat to any terrorist action, civilian guns on planes would be a great deterrent making an actual terrorist incident, and therefore, a shoot out unlikely.