You claim to have some knowledge of debugging, so here are some more questions:
At what stage is the $_POST found to be empty? anytime after the form is submitted...in other words, it never gets filled at all

Is $_GET also empty ? (assuming you're on a page where $_GET is relevant) no - its the admin config page with zone, store name, etc...the edit/save var still works fine, so $_GET is fine

Does this happen to you if you do a similar installation on a live commercially-operated server, instead of self-hosted? don't have one with the same setup to test

Does this happen to you on a different version of PHP? can't really test it

What are your PHP settings related to GPC ? variables_order GPCS GPCS

Do you have a relationship to the original poster, or is your comment another instance of similar problem? another instance...saw the thread, having the same problem...jumped at the opportunity

On a side note, OSCommerce has the same problem.