As far as I know, sitemaps are used primarily to inform the search engine of the pages on a website directly, rather than waiting for the search engine to figure them out from links from external sites. I vaguely remember we used to generate sitemaps, but then stopped because google more-or-less totally ignored them, and instead choose to index articles based on their own algorithms and measures of "importance".
I am sure google already taps into recent changes in wikipedia, but it might be worth contacting them officially to see if edits marked as vandalism can be threated with larger priority in their indexing process.
Cheers, Robert
Anyway, the standard way to do this is Sitemaps:
http://www.sitemaps.org/
As the name suggests, "Sitemaps" were originally intended as hints about site structure, but search engines like Google now use it as a sort of feed of recently changed pages.
http://www.sitemaps.org/faq.php#faq_submitting_changes
They don't accept something sensible like RSS or XMPP even from other top 50 websites, unless you happen to be Six Apart or Twitter. Still, we could ask, since Daniel Kinzler has a working demo of recent changes via XMPP.
Alternatively, we could use the XMPP stream to either transform it to a Sitemaps-compatible structure or generate both kinds of files at the same time. I assume, famous last words, that the really heavy lifting is already done since we have a recent changes feature.
I don't know if I'm committing any resources to this (I'm still busy with other stuff for the next two months at least) but I happen to know a lot about this from an aborted project at another employer, so I have always wanted to actually use that knowledge.