Per https://bugzilla.wikimedia.org/show_bug.cgi?id=24781 , the XML output returned by the API has a namespace since 1.18.

1.17: <api> 1.18: <api xmlns="http://www.mediawiki.org/xml/api/">

Apparently this breaks for some people using XPATH expressions, although comments on Bugzilla indicated this shouldn't be the case, and since I don't know anything about XML namespaces I trusted my fellow MW devs who said it wouldn't be a problem :)

Roan