It was confirmed by Joe on his twitter 10 minutes ago:
@joebelfiore
Yowza, you're going nuts 4 #WP8.1 dev preview. Everyone relax thru weekend, check in early next week. Enjoy Win8.1 update, write apps!
https://twitter.com/joebelfiore/status/454281727158919168
@joebelfiore
Yowza, you're going nuts 4 #WP8.1 dev preview. Everyone relax thru weekend, check in early next week. Enjoy Win8.1 update, write apps!
https://twitter.com/joebelfiore/status/454281727158919168