I'm using a .htpaccess auth for wp-admin, and the upgrade to 2.63 broke all voting - it now sends a request to wp-admin/admin-ajax.php, which returns 401 Authorization Required.
I believe no user-facing non-admin plugin should ever access wp-admin - wp-admin should be reserved for when you're on wp-admin already. Using .htaccess protection is a common anti-hacking technique, so I believe more people will be affected.
I have to revert to 2.62... except you didn't tag it in svn. I'll just restore a backup :(
Hope you can fix it in the future.