I've used TED talks in my classrooms a couple different ways. The first way is more literal--as examples for public speaking. My sophomores analyzed the speech structure of "The amazing intelligence of crows: Joshua Klein" and also the physical and vocal inflections of "If I should have a daughter: Sarah Kay." These and many other TED talks accessible to a general audience show students a spectacular example of a professional speaker doing what they do!
I have also used TED talks conceptually. This year in my AP Literature class, I paired "The danger of a single story: Chimamanda Adichi" with our study of Wright's Native Son and "Dan Ariely asks, Are we in control of our own decisions?" along with Huxley's Brave New World. In those cases, we used our reactions and discoveries from the talks to spur topical discussions that we could connect to the reading.
It's one of my dreams to one day attend a TED conference! The talks are an amazing way to bring the ideas of great minds right into our classrooms, and surely belong in the classroom as something in the toolbox of teaching techniques. It is, as stolperia pointed out, imperative, though, that we take the time to make relevant connections to the clips, though, if we expect students to truly learn from what they see and hear.