It is to my knowledge that when you delete something (not send it to the Recycle Bin), it's not actually removed from your hard drive, but rather marked for overwrite, so it's seen as free space. I've heard there are programs that can actually delete files from your PC, and it would be cool if CCleaner noted these files that were considered free space and actually deleted them.

The newest version does do secure overwrites; it writes random data over what is being removed several times, so that it can't be recovered.

By "newest version", he means the new beta version, not the current release.