It would be great to see MediaWiki generate a Sitemap page readable by Google Webmaster Tools.
I think a good special page to put this feature on is Special:Sitemap. The command ?output=xml shall output the Sitemap for Googlebot to read. Without the command the wiki will output a page (restricted to sysops) that will allow the user to change the articles the wiki will output for Googlebot. I would be more than happy to write the code for the interface.
The layout for the Sitemap can be seen at the Google Webmaster Tools help center.