it's not a WordPress Plugin, but phpList is the best you'll get (especially if you don't want your site account suspended for overemailing past the hourly limit).
There is a bridging plugin for WP standard and phpList - that might be portable to WPMU as a wp-plugin set to site wide as a plugin. I haven't tested it out yet in WPMU but will be doing so sometime in the next couple of weeks.
The beauty of using phpList with WPMU would seem to be that you could create a very simple admin-page info plugin to point your blog admins to your phpList install, if they want to run newsletters, then they could use the existing bridge plugin (that you've installed as a site-wide plugin) to place a newsletter sign-up for their readers on their blog sidebar (as a widget).
when it comes to managing mailing lists and newsletters, and subscriptions from the former to the latter, there really is nothing that beats phpList. The cvs import tool (for mailing lists and duplication sweeping) is awesome and the bounced mail management and database purging is incredible.
Gaz