Bruce Schneier points that filesystems sometimes get in the way of secure file deletion.
I blogged about that six months ago (second point in that bill) after checking my understanding of the question with the developer of Inferno.
I since heard about similar stories quite a few times, either from software like filesystems or recovery systems or from hardware like Flash memory putting the content of a file in arbitrary locations. It seems to be a fairly well known fact among people who spent time on the matter.
To my mind, apart from shredding entire drives when the hardware is disposed of or goes from an user to another, companies should not waste time on shredding.
Of course, I guess Bruce Schneier would argue about encryption, rather than deletion :-)