Recovery of supposedly erased data from magnetic media is easier than what many people would like to believe. A technique called Magnetic Force Microscopy (MFM) allows any moderately funded opponent to recover the last two or three layers of data written to disk; wipe repeadetly overwrites special patterns to the files to be destroyed, using the fsync() call and/or the O_SYNC bit to force disk access. In normal mode, 34 patterns are used (of which 8 are random). A quick mode allows you to use only 4 passes with random patterns, which is of course much less secure.
(man page: http://abaababa.ouvaton.org/wipe/wipe.1.html )