Deanna & Kevin released the WPMU Default User Role plugin earlier this week. In WordPress MU, new users are added to the site’s dashboard blog if the site admin has created one. Otherwise the user is added to the main blog in the WPMU site.
With the default user role plugin, you can specify additional blogs to add new users to and specify a role for the user for each blog when the new user is created. I gave it a test earlier in the week and it works like a charm.