In 2010, Apple’s Platform Technologies Group (a subdivision of the CoreOS department) spent some working on a previously-embargoed project to port the Mac OS X Darwin kernel to the ARMv5 chipset architecture -- the same family of chipsets that currently powers iOS devices like the iPhone and iPad.
Here's a snippet from the project Porting Darwin to the MV88F6281, subtitled ARMing the Snow Leopard:
Apple may have begun testing what appears to be ZFS (Zettabyte File System) support in the latest iOS 5.1 beta. If the information received by iMore is accurate. references to ZFS, including the mounting tools, are contained deep inside the system files. This raises a lot of questions about where Apple may be taking iOS storage in the future.