Apple's registered developers who have downloaded an updated version of the official WWDC app have been able to stream this year's session videos the day they were made. Developers can alternatively access an archive of WWDC 2013 session videos through both the app and Apple's official portal for developers.
It looks like Apple has now published those clips on YouTube for everyone to watch, or so it seems. It was unclear at post time whether the iPhone maker actually operates a brand new account seemingly created for the purpose of hosting these NDA clips...