If you have any kind of Apple developer account, even the free one, you can watch Typedrawers’ own Antonio Cavedoni
as he hosts his own keynotësque video
about the San Francisco family of fonts for the Apple Watch and iOS 9 (indeed replacing Helvetica).
(General WWDC Videos page.
San Francisco as iOS 9 default font was quite openly illustrated
during the WWDC 2015 opening keynote.)