From the official announcement: "Today we're excited to launch Hangouts On Air to Google+ users worldwide. So if you have something to say—as an aspiring artist, a global celebrity, or a concerned citizen—you can now go live in front of a global audience.
With just a few clicks, you’ll be able to:
Broadcast publicly. By checking "Enable Hangouts On Air," you can broadcast your live hangout—from the Google+ stream, your YouTube channel or your website—to the entire world.
Official announcement: http://googleblog.blogspot.com/2012/05/google-hangouts-on-air-broadcast-your.html ;
Find out more: http://www.google.com/+/learnmore/hangouts/onair.
Via Robin Good