Is there any difference between the additional-tags & the Unfiltered MU plugins?
Well, consider that fixed... this plugin simply allows users to add additional tags (embed, iframe, etc) to be used when writing a post or page (in their html tab).
Unfiltered MU gives Administrators and Editors the unfiltered_html capability. This prevents WordPress MU from stripping <iframe>, <embed>, etc. from these users' posts.
The plugin can either be used globally for your entire MU site, or it can be applied on a blog-by-blog basis.