The thing about artists and politicians, etc, can also be done, and has been done with Hangouts. It was also called On Air before these guys had the name. I don't know why Google is still not allowing everyone to use that feature, though.
They are built on TokBox, which seems to be getting some traction as well. Will be interesting to see if it's just one killer app or many that emerge.
Thanks! TokBox provides the video platform, but the OnTheAir team is a small, super-smart group which is tightly focused on nailing the end-user experience. The stuff they think about - everything from testing how to promote shows to where the users' eye-level is - has been incredibly thorough and thoughtful. We've been really impressed with them.