This was recently the subject of an article in "Network News". They tested various methods, then submitted the disks to a data recovery company to see if anything could be recovered. The answer, to summarise, was that you cannot completely erase a hard disk using software methods - even low-level format. [Reason : the disk will have almost certainly remapped some sectors, leaving some sectors subsequently inaccessible to the "erasing" software].
The easiest method they found to completely erase the disk was to remove the platters, and take a sander to them.
[ 31 October 2001: Message edited by: malanda ]