WP3.1 multisite "mu-plugin" that allows you to add a user to all sites or no sites. Just drop in mu-plugins.
This plugin will give you the ability to pull up any (non-Super Admin) user and in one click add that user(with a global role) to all sites in your MU install. Likewise, if you want to remove a user from all sites it is one click as well. CAUTION: WordPress has an optional mechanism when deleting a user from a site that will allow you to assign that users posts to another user. I have ignored this for this release. Please take a DB backups until you are used to this plugin and Multi-Site.