From what I can tell, when you delete files on a computer, they aren't wiped from the hard-disk, they're just renamed and considered to be free space. If this is correct, are there any programs around that can find and recover files that you have deleted and want back? If so, I really need to find one!!
Basically what you are saying is true. A flag in the file header is set for deleted files. The "deleted" files may be retrieved as long as you have not run disc defragmenter or other intrusive utility. If you have a PC then try getting hold of Directory Snoop V3.2. This is extremely powerful.
In addition to the previous answer, the files may not be recoverable if you added new files to your system since you deleted the files you wish to retrieve.