Apple to Use San Francisco Font for iOS 9 and OS X 10.11
Apple is planning on switching the main font of iOS 9 and OS X 10.11 to San Francisco, says 9to5Mac. Currently, Apple uses Helvetica Neue as the main font for iOS 7-8 and OS X Yosemite. San Francisco is used as the main font for the Apple Watch. The only problem with this rumor is that it's going against the overall look of the operating systems. San Francisco is a much thicker font than Helvetica Neue. Apple wanted to go with a more simple look with iOS 7 and it seems a little strange that they'll pick a thicker font for iOS 9. Above you can see a comparison between the current system wide font and the rumored one to be announced in three weeks.
While it seems a little strange, this does make sense because this font was developed by Apple and where better to use it than in your own operating systems.