I've found and just used a program which simply writes 'zeros' into each and every memory location on the card.
It's a dangerous program though because it lists all disks and partitions and all removable media on your system and you have to be very careful to select the correct one. When it is done it has wiped the card completely clean and ready for formatting. I think that is about as low level as you can get.
Walt

[This message has been edited by Walt M (edited 01-25-2008).]