I suggest using Disk Utility (from the install disc) and doing a "zero all data" instead of a seven-pass wipe. The multiple wipes can take forever and a day, and the added security is usually very little and not worth it for a regular user if they're sending their machine back.
-Chasen
I understand the concern, however, because people have reportedly received reconditioned goods from Apple (Mac, iPod) with previous owners' data on them.