WP3.0 multisite “mu-plugin” by D Sader, allows SuperAdmin to set the “Primary Blog” (aka Primary Site) of a user while editing a user’s profile.
Can I set the Main blog as a user’s primary? Yes.
Can I set the Dashboard blog (if enabled) as a user’s primary? Yes.
Can I set Donncha’s Sitewide Tags blog (if enabled) as a user’s primary? Yes.
Can I add a user to some other blog “special blog” as their primary? Yes, but see “special blog” comments in the plugin code.
Does this plugin filter the list of blogs already listed at a user’s Dashboard->My Sites->Primary Site? No.
Download and info: http://wordpress.org/extend/plugins/primary-blog-switcher-for-superadmins/
