This is such an old wive's tale its not even funny.
There is nothing in an airport security scan that will harm your computer.
First of all, the X-ray scanner will in no way harm your laptop as it is merely a different wave of light (more or less).
Secondly, the metal detector (the only thing that in any remote way that would affect your computer as it deals with magnetic fields) has such a low magnetic sensor, and that magnetic sensor only waves over the laptop for such an ultra brief period, would not have any affect on your computer.
Now, if they jacked up the magnetic force to the absolute maximum level, then let your PowerBook sit under it and "cook" for say like....2-3 hours....then maybe, just maybe, the magnetic field generated would cause some damage to the HD.