It seems to me that a lot of the debate around aviation security is that we (generally) are conflating two separate questions:

1. What measures are appropriate to ensure the safety of the passengers on the plane.

2. What measures are necessary to ensure that the plane is not turned into a weapon against others (i.e. crashed into a building).

So long as we have secured cockpit doors, it seems that #2 is largely resolved. "Another 9/11," with a group of men armed with knives/box cutters couldn't turn a plane into a giant cruise missle. They could, however, do a lot of harm to the passengers on that plane.

It seems to me that there is a genuine national security interest in mandating safety precautions to block #2, since that sort of attack affects people who have nothing to do with the flights themselves. In terms of #1, however, airlines should be allowed to choose whatever level of security they want to pursue (i.e. screen for bombs or don't), and let the passengers decide.

So, this would lead to a Federal mandate banning carrying on items that could credibly allow passengers to breach the secured cockpit door (i.e. no SWAT solid-slug shotguns, no sledgehammers, no crowbars), but knives (which could in theory wreak havoc in the cabin, but can't seize the plane) and liquids (I know there's debate over this, but for the sake of argument, assume they could be used to make a bomb) would be allowed, as they only present a risk to the passengers on the plane.