The “WWDC 2015” in the invitation is typeset in San Francisco Rounded. The legal text on Apple’s limited edition hardware and its accompanying regulatory filing are also set in San Fransisco. What’s the over/under on iOS 9 and OS X getting San Fransisco as a universal system font?
(The one thorn in this theory is OS X: it just changed to Helvetica Neue. Would Apple do two system font changes in two years? I don’t necessarily think they’d be dissuaded from it; I suspect the main reason OS X doesn’t use San Fransisco today is because it wasn’t finished in time, or they wanted to debut it on the Watch.)