On the StudioDell site where they have all their DellTV-ish how-to's and infos, they've got a new 5 minute video called: "Linux 101: What's all the Fuss?" It focuses heavily on Ubuntu with screencasts to go with the commentary that roughly explains the concept of open source and gives examples of popular open source applications.
http://www.dell.com/content/topics/topic.aspx/global/shared/corp/media/en/studio_dell?c=us&l=en&s=corp