Thanks a lot for yor careful explanation.
Unfortunately my problem persists.
Yet, maybe I am focusing...
The xsl transformation works correctly on Oxygen, my XML editing 
software; à è é ì ò ù are all parsed correctly.
Yet when I open the XML page in the browser (Safari on Mac Os X) the 
accented vowels turn into strange symbols.
I need to use my xml document inside the browser, since I use it to 
generate dynamic content...
For some obscure reason the document doesn't work at all with Firefox 
and Camino; yet it is a very simply structured xml document... When 
opened with this browsers (both of them use the gecko rendering engine) 
I get an error message saying that the xsl document can't be found (but 
I assure you it is there!)
I am afraid something is missing in the prolog; I attach again the 
prologue of xml and xsl files.
As I specified before, the xml file is actually a php file resulting 
into an xml document


XML:

<?
header("Content-type: text/xml");
print '<?xml version="1.0" encoding="iso-8859-1"?>';
print'<!DOCTYPE inferno SYSTEM 
"file:/Users/davidebenini/Documents/Universita%CC%80/Dottorato/inferno/inferno.dtd">';
print '<?xml-stylesheet type="text/xsl" href="inf-dev.xslt"?>';
?>
AND HERE THE ACTUAL XML CONTENT FOLLOWS

XSL:

<?xml version="1.0" encoding="iso-8859-1"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
version="1.0">

Any suggestion is welcome.
Thanks again