The Darwin kernel has been ported to the ARM architecture. For those not familiar with Darwin, it’s the core operating system that lies under both Mac OS X and iPhone iOS. It’s the true core foundation that bridges the kernel to say, SpringBoard/loginwindow/etc.
The system root filesystem is based on iPhone iOS 4.3.5. It runs everything flawlessly for the most part. However, there are a ton of kernel bugs to fix, including power management.
Feel the urge to tinker? Run Darwin on say a Nokia device (shown above)?
Source code is available on GitHub.