The standard xmlrpc.php script is quite commonly installed with many free blogging. Unfortunately, it is quite easy to exploit and often abused by amateur 'hackers' and spamming groups for malicious purposes.
However, most users that have the script on their sites don't actually use it, therefore we have decided to block access to that filename to protect those users.
If you wish to use an XMLRPC service, simply rename the file to something like 'xmlrpc_post.php' and reference that script instead in your posting service.
For those using the popular blog application, Wordpress, there is a plugin that has been developed to assist with applying the above changes. This can be found at http://wordpress.org/extend/plugins/rename-xml-rpc/ or by searching 'Rename XML-RPC' within the plugin section of your Wordpress dashboard.