After quite a bit of work, WP-OpenID is now ready for WPMU! Always wanted your WPMU install to consume OpenID for logins and comments? Now it can!
I am already happily running this modified version on blogs.gnome.org and perkypants.org (WPMU and WPSO respectively), in order to test the changes in both environments.
To try it out, download the original WP-OpenID plugin, and follow the directions in the bug report. Both are linked below.