Why do we have to struggle looking for a solution to the problem when WordPress can fix it permanently through code in xml-rpc.php? Lines 25 through 27 of the code says:
// fix for mozBlog and other cases where '<?xml' isn't on the very first line
if ( isset($HTTP_RAW_POST_DATA) )
$HTTP_RAW_POST_DATA = trim($HTTP_RAW_POST_DATA);
Or does this part of the code work? I believe that even if we find a temporary solution, xml-rpc.php will still generate an invalid feed if this part of the code is not fixed.