I'm not sure what planes being improperly registered has to do with flying them into things. All four planes that crashed on 9/11 were properly registered. Was the plane that crashed into the IRS building unlicensed?

If I had to guess I'd say the ratio of crimes committed in properly registered automobiles vs. improperly registered aircraft is somewhere around a million to one.

Ben, being a licensed pilot, might find this one interesting. The Associated Press discovered that the FAA's registration system for planes is so badly messed up that 119,000 planes are not even properly registered.

ABC news did a check; a plane known to be dismantled and being sold off for parts was listed with the FAA as being active and in service. The FAA has generally operated "on the honor system" and this has brought back fears of misuses of the system such as someone buying a plane and using it for constructive purposes, like the guy who crashed a plane into an IRS office earlier this year.