On the other end of the spectrum, I would be happy if I could install a wiki and share the login credentials between WordPress and the wiki. I hacked MediaWiki a while ago t
Both MediaWiki and Wordpress support OpenID:
http://www.wordpress.org/extend/plugins/openid/
http://www.mediawiki.org/wiki/Extension:OpenID
Though, I think for automatic logins (after you log in to one, you automatically log in to the other) you would need to look into implementing checkid_immediate
http://www.openid.net/specs/openid-authentication-2_0.html#anchor28