Speaking of 10.10, Gruber notes:
I expect the system font to change for the first time since Mac OS X 10.0 back in 2001. Helvetica Neue is the obvious choice, since that’s what iOS uses. The wildcard would be Apple Sans (perhaps with a new name), a new typeface Apple has been designing in-house for years. (And if OS X switches to Apple Sans, maybe iOS 8 will too.)
I assumed Myriad, based solely on the typeface used in the iOS 8 Banner — the long rumoured Apple Sans hadn’t even occurred to me.
Much like Maps, Apple used a third party for as long as it made sense, then switched to an in-house solution. The same could be true for their system typeface.