This plugin removes the nag notice at the top of WPMU plugins and themes.
Q: Will this always work?
A: It should work with all WPMU themes and plugins, but no promises, especially if they update how the notifications nag is displayed. I'll try and update it.
Q: How do I thank you?
A: A little cash. http://www.protospace.com/wpmu-no-nag-plugin/