When beginning the iPhone project, Apple realized they'd be better off adapting their great operating system, Mac OS X, than starting from scratch.
The iPhone has now been out for over a year and iPhone OS 2.1 is being prepared for release. It may not yet be a mature system in the sense that it won't continue to change in major ways, but it's at least stabilized, shaped to fit the device and environment it must serve.
While it shares a lot with Mac OS X, it's a distinct system, the design of which has been driven by the combination of a touch UI in place of a keyboard and mouse and limited resources otherwise.*
Given those design parameters have more in common with the wide range of devices which are apt to come tumbling off the drawing board soon than do traditional desktop and laptop computers, one has to wonder to what the iPhone OS might itself be adapted. The hard work of paring down Mac OS X to fit into a small footprint has already been done; it's a superb starting point for further development, and if it needs elaboration, there's a great, compatible code-base in Mac OS X to draw from.
In the end there may be a family of OSes, all using essentially the same kernel and core components, but residing in very different devices, as well as running on an array of different processor architectures. As did the iPhone, each of these devices will bring new pressures to bear on the design of the OS, but, as applicable, all will reap the benefits. It's a strategy that promises not just new and interesting gadgets, but further refinement of what is already a great operating system.
* It's maybe worth noting that the iPhone is no slouch as a computer. It's Flash memory is comparable in size to the disk drives in early iMacs (but faster), and it's processing power and RAM compare favorably with those machines.