Just to expand on what Corona688 has already said: i once read that some (military) paranoids don't stop here but erase disks several times (IIRC 7 times) with different hex values to make sure the overwritten bits can't be restored by some obscure means. If you want to do this prepare a stream with "0xFF"s quite like the "0x00"s from "/dev/zero" and use these two streams alternatingly to write the disk several times.

How often you do that is determined by your own level of paranoia. ;-))

I hope this helps.

bakunin