Helvetica Neue took over as the system font in iOS 7, and last year, it spread to OS X Yosemite as well. However, the Apple Watch didn't carry over that trend, using Apple's own San Francisco font on the tiny, wearable screen.
And now it seems that the Watch was a testing bed for San Francisco, as 9to5Mac cites inside sources in a report saying it'll head to iOS 9 and Mac OS X 10.11, both expected out this autumn. In fact, we'll probably get a glimpse at both next month at the Worldwide Developers Conference. The mocked-up screens below show what the font should look like running on an iPhone.
Not everyone has been a huge fan of Helvetica Neue, as the slim weights of the font can affect readability. San Francisco looks great on the Apple Watch screen, and Apple has said it was designed "specifically for legibility." That ought to carry over to larger screens, as well.
This year's iOS update has been pegged as being more for stability and performance rather than a big aesthetic overhaul, so a font change would be a subtle, yet not dramatic visual tweak. Apple has reportedly had the font in builds of both iOS 9 and OS X 10.11 since late March, so it seems well on its way to making the consumer version.
As 9to5Mac points out, even the recent 12in Macbook uses San Francisco as the letter font for its keyboard, so a shift to using it across all operating systems (and presumably hardware, as well) doesn't seem so far-fetched. We should find out for sure come early June.