I wouldn't bet on it. There's too much metal in the case to count on the magnetic field wiping out the bits on the platters. Physical destruction is the only way to be sure. The software suggested above to write multiple ones and zeros over your data is good enough unless the CIA thinks you are important enough to rewire your hard disk or look at the platters with an electron microscope.
Well thanks for the information.
I don't care for myself, but know lots of doctors who have business computers and they are not allowed by law to share patient information.
So far I have all such friends destroy their hard drives since so much potential damage to patients could be had.