Note that $_POST is NOT set for all HTTP POST operations, but only for specific types of POST operations. I have not been able to find documentation, but here's what I've found so far.
$_POST _is_ set for:
Content-Type: application/x-www-form-urlencoded
In other words, for standard web forms.
$_POST is NOT set for:
Content-Type:text/xml
A type used for a generic HTTP POST operation.