No, seriously, Daniel Ehrenberg has implemented an XML-RPC client and server in Factor; the code is in darcs,
contrib/xml-rpc. Good stuff! After I release Factor 0.87 I'm going to get back to work on the web framework, and cook something up so that web apps automatically have an XML-RPC interface. I'd also like to put together a demonstration app that uses the XML-RPC client to make pastes at Lisppaste.