> tightly controlled and still been somewhat 'open' (see Windows vs Mac).
Huge chunks of OS X are open source. It's not necessarily equivalent to throw it into the same box as Windows (there are no "open" forks of a Windows based OS, but there are of Darwin, and FreeBSD has benefited from Apple as well.)
> Isn't Darwin open source in the same way Android is?
Not really. Android is a complete system, with a kernel, libraries and the whole GUI on top of that. Darwin is just the kernel, and I think they've sort of phased out doing much with the 'open source' aspect of it.
Huge chunks of OS X are open source. It's not necessarily equivalent to throw it into the same box as Windows (there are no "open" forks of a Windows based OS, but there are of Darwin, and FreeBSD has benefited from Apple as well.)