Userthemes Revisited
I've tinkered with the original a bit, again.
Adds ability for enabled users to move system themes to their file blogs.dir/id/themes/ folder and edit their themes if you allow access to the theme editor. Access Managed via Site Admin.
Practical uses for me: I can edit themes as I am accustomed in my single WP install. I can switch between specialty themes without logging in as a SiteAdmin.
If a particular user wants a unique/rare/premium theme, I can upload it to their blogs.dir/blog_id/themes folder and only they can ever see/edit/activate it.