I whipped up a little extension for MediaWiki that implements the MicroID standard (http://www.microid.org/ ).
http://www.mediawiki.org/wiki/MicroID_extension
MicroID is a teeny-tiny standard that lets users with accounts on multiple Web services assert that those accounts belong to the same person. The standard consists of a single <meta> element with a hash of the user's email address and the address of the current page. MicroIDs are lightweight, relatively hard to spoof without control of the server, and help assert identity without sharing private information.
The extension I wrote adds a MicroID <meta> tag for User: pages for registered users with confirmed email addresses. The user can set whether they want to show their MicroID or not in the "Misc" tab of the Special:Preferences page. (Someone using MicroID to verify a user's identity has to know their email address already first; however, some users may not want to share their identity in any way.)
The extension has, as far as I can tell, little performance impact. It loads the user record for the user page, but since that's loaded anyways for user pages there's not much cost.
The feature is rolled out on Wikitravel and we've had some positive feedback on it. It would probably be a useful extension to add for Wikimedia projects.
~Evan