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
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Any updates on this? I have exactly the same plroblem over here...
The API's a work in progress, so it's probably just not ready yet.
Take a look at WikiXMLDB - provides a way of querying Wikipedia in XQuery (XML query language).
http://wikixmldb.dyndns.org/