Wikipedia API

Wikipedia (as MediaWiki) has a couple of APIs for querying and fetching other data: query.php, api.php.

I was trying to fetch an OpenSearch query, but can't get the response in XML:

http://en.wikipedia.org/w/api.php?action=opensearch&search=CD4&format=xml

Fetching information for a page by title works, anyway:

http://en.wikipedia.org/w/api.php?action=query&titles=CD4&format=xml