In the current development version of Serendipity, we have unbundled the XML-RPC API functions from the release version and made the functionality to post entries via XML-RPC (MT/Blogger API) calls available as a additional plugin called Post via XML-RPC (serendipity_event_xmlrpc).
The reason for this is that very few people use XML-RPC posting to our experience, and it is a inherent security risk to have this functionality available if you don't use it, as the past has proven. To overcome this possible vulnerability, you need to now actively install the mentioned plugin to make XML-RPC posting available. Sending and receiving trackbacks is NOT affected by this, only the "Server"-Part of that API is.
The URL for the API endpoings will not change; if you have not installed the plugin, you will see an error message displayed. Outsourcing this functionality as a plugin allows the Serendipity Team to respond easier to new issues with the plugin and make enhancements to the XMLRPC module.
A general advice for Serendipity 0.8.3 users is to remove the serendipity_xmlrpc.php file if you do not use XML-RPC entry posting.