It's this bug: http://bugzilla.wikimedia.org/show_bug.cgi?id=11428
Just add "$wgUsePathInfo = false;" to your LocalSettings.php for now.
http://www.mediawiki.org/wiki/ Manual_talk:Short_URL#Problem_with_http:.2F.2Fwww.mediawiki.org. 2Fwiki.2FManual:Short_URL.23Rewrite