Investment banking firm Jefferies stated the obvious this week when it issued a report predicting that iOS and Mac OS X will be one operating system by 2016. Nearly a year ago, Apple CEO Steve Jobs said that was his goal in what I playfully dubbed his MiOS strategy. Jobs had just previewed Mac OS X Lion, touting the user interface capabilities it was borrowing from iOS. The forthcoming iOS 5 also takes some UI concepts first released in Lion, but as I'm under NDA with Apple, I can't tell you which ones.
iOS is based on a subset of Mac OS X, so in a very real sense, they always have been the same operating system. As horsepower has improved in mobile devices, Apple has enlarged iOS to take on more of what the desktop Mac OS X could handle, such as more multitasking and more complex graphics and video processing. At the same time, Apple has been steadily pushing gesture-based peripherals -- not just its laptops' gesture-capable touchpads, but also its Magic Mouse and Magic Trackpad that bring gesture savvy to any Mac -- in a gentle but persistent reeducation of its Mac users.
Over time, you can expect iOS and Mac OS X to borrow more from each other, keeping distinctions appropriate for their individual use cases. You can expect a continuum of capabilities as the two OSes eventually merge into MiOS (I doubt Apple will call it that, though).
Microsoft is following Apple's strategic lead
The real news here is that it's not just Apple that sees the distinction between a desktop OS and a mobile OS. Microsoft is making exactly the same bet with "Windows 8," its code name for the successor to Windows 7. Windows 8 adopts the generally well-regarded Windows Phone 7 UI, called Metro, and is supposed to run on both computers and tablets. Apple will get there first, as its two OSes already share much in common, whereas Microsoft has two decades of Windows legacy to carry around. Even Windows 8 will run Windows 7 under the hood for backward compatibility -- not just the binaries but even the UI if desired.