Update: According to iMore's Rene Ritchie, Apple confirmed that the videos are not official and, as suspected, in violation of the NDA.
After the release of the keynote's video on its website, iTunes, and YouTube, WWDC 2013 session videos have surfaced on YouTube under the account WWDCVideos.
Originally tweeted by Safari and WebKit engineer Timothy Hatcher, the WWDCVideos channel has the following description:
Get an in-depth look at what's next in iOS and OS X, and learn how to take your apps to the next level. With over 100 sessions, extensive hands--on labs, and engaging events, you'll connect with Apple engineers and fellow developers for an incredible week of inspiration.
If confirmed official, this wouldn't be Apple's first YouTube channel, as the company has been uploading keynotes, promo videos, and TV commercials on the Apple channel for years now; it's unclear, however, how such videos would be available publicly on YouTube considering Apple's NDA, which prevents developers from discussing upcoming iOS and OS X features publicly.