Now that the enormity of the terrorist attack has had time to sink in, the question is, "How will this affect our lives?". Principally, I believe we will only be directly impacted by how air travel security is changed.
In the past, security has been associated with passenger safety. Airport security has been focused on preventing potential hijackers from getting on the plane with deadly weapons. No weapons, no hijacking.
In the very rare event that a weapon was smuggled on (only once in 13 years) and a hijacking attempted, the pilot flew the plane to whatever destination the hijacker selected to avoid possible harm to passengers. All that has changed.
The lesson from the September 11th hijackings is that the hijacker (s) can never again be allowed to obtain physical control of the plane. The plane could then become a flying bomb with the lives of all aboard PLUS the lives of those at some potential ground target now at risk. The lives of those aboard will now be considered as a second priority to avoid another World Trade Center catastrophe. No matter what the circumstances, a hijacker will never again be allowed to enter the cockpit.
By his actions instructing that plane #4, the one that crashed in Pennsylvania, be shot down if it appeared to be heading for Washington DC, the President indicated to all that hijackings will never be allowed to succeed. A new standard has been set. This will pretty much eliminate any more such acts.
Let's review the history of domestic hijackings for a minute. The D B Cooper type of hijacking for ransom, where the hijacker parachuted from the plane to escape, was totally eliminated when it was made impossible to open the rear stair exit in flight. No way to parachute. Then there were the hijackings to Cuba. Castro started putting the hijackers in jail. No more hijacking to Cuba. Now the President shows he will authorize shooting down dangerous hijacked planes. Who's going to try this again?
To minimize the potential for a hijacking, there will be significantly increased security exercised over boarding passengers. Their safety will be increased to compensate for the added risk they will face if the security system fails. I can see a prohibition against passing anything, including laptop computers or cameras, around the X-ray machine. Spray cans of cosmetics or anything else may be prohibited also.
The cockpit will have to remain secure even if passengers are being assaulted. There are some mitigating measures that can be taken to protect the passengers even then but not without some additional risk. The passenger cabin can be filled with a sedating gas to render everyone unconscious. If the hijacker was ingenious enough to smuggle a gas mask aboard, the cabin could be depressurized. This would render everyone unconscious from lack of air. I doubt a hijacker could smuggle an oxygen canister aboard (don't overlook medical oxygen) without careful processing.
Unfortunately, some elderly or children might be harmed from the gas or lack of oxygen for a short period. There would be risk but not too much considering the alternative.
The prioritizing of aircraft security over passenger security will remain invisible and unspoken of until such time as there might be a hijacking attempt. Only then will the flying public realize their second place status. Airport security will substantially increase at least until three or four years pass with no significant incidents. Then it will start to degrade again.
But on the bright side, the new security arrangements will so lengthen the normal home-to-destination travel times that business travelers will turn more and more to teleconferencing. This will reduce the demand for air travel but at expense to the recreational traveler. Without the business passenger paying top fare, others will see their average ticket price markedly increase.
Ironically, with the shoot down policy in place, the expanded security procedures are moot.