Hi.

There is definately no opportunity for the data to stay on the drive forever, however, really, the data erased is not gone from HDD momentarilly, and it can be restored using special data recovery tools. There are ways the data can be erased completely, some utils perform this operation. I can say Boot Disk data tools set CD image contains utils for both data recovery and destruction. If you are interested, you can give it a glance.
http://www.ntfs.com/boot-disk.htm