Apple's Disk Utility, however, does not seem to wipe file slacks when wiping free space, resulting in an incomplete erase of free space. File slacks is the space after a file's data ends and until the file system block ends, and on a modern harddrive it can constitute gigabytes of data. Forensic investigators frequently find interesting information in file slack.
To wipe free space AND file slack, you need to use Jetico's BCWipe.
For a tutorial, see
http://wipeosx.wordpress.com/