There are no guarantees that using a magnet will delete all of the data off of a drive. DoD requires that if you are going to use a magnet to wipe a drive the magnet has to be able to fluctuate the power of the magnetic field to insure the data is wiped. With a normal magnet that you cannot change the magnetic field, you may end up leaving a lot of data available for a forensic scientist to find. Degaussing is much more effective than a magnet, but again, you have to use a degausser that is specifically designed to wipe a harddrive.

Here is a DoD memo with guidelines on destroying data: www.doim.army.mil/a2rmp/docs/harddrives1.doc