Apple has been slowly moving to Helvetica over the years. At WWDC 2014, it made Helvetica Neue the default font in OS X Yosemite.
However, it shouldn't be any surprise. This was in the making. Craig Hockenberry of Icon Factory accurately predicted the change way back in April. I saw it as an impending change ever since the release of iOS 7's beta. iOS is teeming with Helvetica Neue. There was a major controversy with the introduction of Helvetica Neue Light. Then they tuned their fonts again to get it just right. Apple adorns Helvetica Neue with pleasure on every device.
A lot of type enthusiasts (like myself) and type professionals (like Tobias Frere-Jones) have been clamoring about Apple's new system wide font Helvetica Neue. But, I have to admit: I am a bit perplexed by the thinking being employed. I agree, Helvetica doesn't have the best performance at smaller typefaces. It strains your eyes. It can be confusing to read similar characters. It's unpersonable and bland.
And it makes absolute sense.
Apple was the first to push forward with high resolution displays and they are continuing to push that trend forward. Retina, retina, retina all the way. At one point, @2x will become @1x. The system-wide font is another push into this territory. By applying Helvetica Neue system-wide, Apple is making its strongest commitment to high pixel densities to date. In the (very near) future, there won't be any non-retina devices in Apple's line-up. It makes no sense. Helvetica Neue is setting the stage for stronger advances in typography (even as a century old inspired typeface).
But you don't have to hear it from me. Apple's own marketing copy says it all (emphasis mine):
For some, the choice of a font may not be a big deal. But to us, it’s an integral part of the interface. In OS X Yosemite, fonts have been refined systemwide to be more legible and consistent across the Mac experience. You’ll notice a fresh, new typeface in app windows, menu bars, and throughout the system. And it looks absolutely incredible on the Retina display.
Kudos Apple. You've got it. And who knows? In the coming years, we might see Apple Sans.