The "darwin layer" in iOS is fairly inexistant. When have you started a shell on a non-jailbroken iPhone? ;)

I actually meant all the lower layers including kernel and drivers etc. Which do exist, and have not been released.

http://en.wikipedia.org/wiki/Darwin_(operating_system)

"Darwin is now only available as source code,[4] except for the ARM variant, which has not been released in any form separately from iOS."