What there are, of course, are the Apple/*BSD drivers that ATI must provide if Apple is going to use their hardware. Simplest way for ATI to handle this is to just contribute to the OS BSD drivers and make sure they pass Apple's muster.
While Apple continues to release closed, OS unfriendly, lawsuit-as-a-business-model software, their choice of a BSD OS as the basis for their system has had some positive consequences from those of running actually Open systems.
I'm afraid I can't let that slip. Device drivers are substantially different from operating system to operating system, as I'm sure you're aware. But XNU, Apple's kernel for OS X, is not "BSD". OS X is "BSD" by way of a BSD compatibility layer for user space applications, similar to how NeXT before it, and mach before that, to shim in support for a larger set of existing programs. So for most cases, OSX is BSD enough.
Video drivers are a difference case, however. They need direct access to several aspects of their hardware for efficiency and correctness's sake. And how they are written differ in many ways. Apple's system is C++ (http://en.wikipedia.org/wiki/I/O_Kit) based, which is radically different than BSD drivers just based on language alone. What you're suggesting is the equivalent of improving the Linux nVidia drivers by making their Vista drivers open source.
But hey, if you can convince Apple that BSD drivers will help OSX, more power to you.
Justin Dugger