I don't think there is any one thing to do, it's a variety of things that will offer the best protection. On commercial flights, arming the pilots, strengthening cockpit doors, having a camera directed to the passenger cabin so the crew can see what's going on and increasing our ability to keep explosive devices out of the luggage are all needed. Air marshals on every flight would also be great, but I doubt they will ever have enough manpower to cover all flights. However covering as many as possible could not hurt.
My biggest worry as far as aviation would be cargo craft, private planes and perhaps international flights from Mexico or Canada. Also sabotage from the ground or using a shoulder fired missile are possibilities. But missiles such as the SA-7 use a small warhead and hopefully would not have the punch to take down a full size airliner. Even if it took out one engine, the plane could make an emergency landing, i.e. the recent situation in Iraq. Hopefully the warhead would not have the power to do significant damage to the critical structures.
I also worry about the use of business type planes that may appear legit but are not. If they have that level of connections to get all the necessary licensing, etc, then they could sneak up on us that way. Hopefully we have been paying attention to this kind of stuff.
Still, I feel the most likely means of attacking the US at this point would be on the water or through the traditional car bomb type stuff. Many major US cities have water access of some sort and many boats could carry a large enough charge to do major damage if it got close enough to the intended target. I also feel the waterways are most vulnerable and difficult to secure. And when you look at the damage and carnage many of these car bombers have done recently, you can imagine what 20 or so attacks simultaneously could do.
We are all thinking of Al-Qaida using some sort of super sophisticated method of attacking us. But using a more simple approach in big numbers could be just as much deadly if not more. I honestly don't think they are that sophisticated personally. What have they done to show that they are such? Septemeber 11th seemed sophisticated but the truth is we should have and could have stopped that dozens of times had people been paying attention and not been caught thinking we couldn't be hit. Now we are ready, the military and police are ready and most of the citizens are at least aware of the threat. IMHO, the liklihood they could pull off another 9/11 type attack using the same means would be very, very difficult. Airport secure, while certainly not perfect is much better. The cockpits are better protected by armed pilots, stronger doors (with no flight attendents who have keys to open them) and most passengers in this country post-9/11 are automatically conditioned to fight back now when faced with hijackings. So if they hit us I really feel it will be in a different manner. But personally, this may all just be misinformation put out by Al-Qaida just to shake up the people of this country in an effort to spoil our holiday. I will be alert and prepared in case something happens, but I am not going to allow myself to get too caught up by these threats.
-CH