Great news! There's really no reason not to do this. Thanks Apple!
Actually, there's a reason... When they don't do it, all forums, blogs, websites, etc... try to cover the event, which make a lot of media coverage. They probably would have less if they do have a live stream.
But since a LOT of people are waiting for this keynote, they probably convinced enough the media coverage on blogs, forums, websites, etc... will still happen (and of course it will).