These pages show some cool stuff about PDML, a 'language' (though SGML- or XML-dialect would be better) to mark up your documents in a common way (HTML) and let it output as PDF. Note that Internet Explorer renders the source files as text/html, while they are sent as text/plain; stupid IE. View the source of the source files, if you're using IE and interested in the source.
The PDML manual copied here for quick reference for myself.
Combining this with server side XML and XSLT is way cool, IMHO.
AddType application/x-httpd-php .pdml ;-)