I had the good luck of asking these questions with the 'Genius Bar' here at the Apple Store in Tysons. Here is the scoop on "openness":<P>
The core kernel for MAC OS X is Darwin and YES, it is opensource. It is extensible, expandable, scaleable. It is as much UNIX as any UNIX. In an nutshell, Apple has evolved to a UNIX core. The difference lies, primarily in the graphics and user interfaces.<P>
Command line Darwin is just like command line in any UNIX. You can rebuild the kernel, add revise drivers, etc. However, there are licensing issues with what you do for these in a commerical environment. For fun and for your company or for yourself, you can modify the kernel to your hearts content.<P>
The graphical user interface is Apple controlled (proprietary). In a traditional UNIX environment, think of this as replacing X (and X Windows, etc.) with the Apple graphics environment. This is what you pay, super programs like the MAC user interface (on top of the UNIX core Darwin), iMovies, Click-Time, etc. all on top of a multi-tasking, memory management, etc. UNIX core. <P>
In a nutshell, MAC OS X is the marriage of UNIX (the core open system) with Apple graphics (it is more complex than that, but that is the bird's eye view). If you love to run UNIX-based software, you can and you can do this side by side Apple graphics and applications. <P>
So, the core UNIX (Darwin) kernel is open and extensible. The MAC graphics and super desktop applications are not. This is a great marriage; UNIX at the core and Apple at the graphics, application level. That is it, in a nutshell. I'm more excited every time I play on OS X. I now plan to buy an OS X system and replace my Windows desktop completely with UNIX and Apple, running Microsoft applications like Word and Powerpoint in the Apple environment. This is promising and a giant leap for UNIX, IMHO.<P>
However, not sure about security, user management, logins, etc. The systems here boot without a login prompt. Will have to check into how OS X works in a multi-user environment. Still learning ......