Something like 9/11 happening again are extremely remote (I never say never). Why? Security measures in place currently, while not perfect, would thwart weapons (even the box cutters) getting through. Even if they had accomplices (cleaning crew, maintenance workers) leaving the weapons in seat back pockets, cabin crew doors are locked. Besides, after 9/11, no passenger group would ever allow it again (a la Flight 93). And another thing, the flights on 9/11 were almost empty. Flights these days are packed, increasing the passenger to potential terrorist ratio, and making a successful hijacking even more remote.
That leaves cargo. And while it is an area where we are lacking when it comes to security, they wouldn't waste their time (they being the sob's who pulled off 9/11). As they have proven with 9/11, the earlier attempt to bomb the WTC, last August's plot, and the London subway bombings, they want to go for maximum impact and death toll. Meaning they would want to time the bombs to go off over big cities. They could get the packages on planes, but trying to get specific packages on specific planes flying specific routes to get that maximum impact would require a coordination that would be rather difficult (IMHO) to pull off, and highly likely to be detected.
I fly twice a month. I am not worried. I have faith in the American aviation industry.