It would be useful to define an XML namespace for any XML-Formatted output of the web API. This would allow us to embed elements from that output in other XML documents easily. My own use case is embedding <rc> tags in XMPP stanzas.
I would suggest to use the URI http://www.mediawiki.org/xml/api-0.1 for the namespace. In practice, this would mean that the top level <api> tag returned by the API would become:
This should have no impact whatsoever on backward compatibility, except perhaps for clients processing the XML using naive regular expressions. I believe it would be ok to break those, since they rely on things that are never guaranteed in XML.