I've just looked at the new Google Friend Connect plugin for Wordpress and its very cool. Userdata actually gets registered in your Wordpress user database like facebook connect.
It needs a few simple modifications for Mu but its much simpler than the sociable plugin for facebook connect.
The down side of this plugin compared to facebook is it allows people to login through their yahoo accounts which opens it up spammers.
Maybe add the following after line 80 of the server_code.php file:
update_usermeta( $wpid, 'wp_1_capabilities', array('subscriber'=>true) );
update_usermeta( $wpid, 'primary_blog', 1 );
update_usermeta( $wpid, 'source_domain', $current_site->domain );
Here are some links:
Demo
http://ossamples.com/plugins/wordpress/?p=1
Download
http://code.google.com/p/google-friend-connect-plugins/downloads/list
Wiki Install
http://code.google.com/p/google-friend-connect-plugins/wiki/WordPressPlugin