If Apple has extended the development of xnu in a similar fashion to the FreeBSD kernel, then the platform specific code will be focused in a single location, and be incorporated in such a way that the vast majority of the kernel source code is platform agnostic.
At that point, it only matters that you don't have the i386 code in very odd scenarios, but otherwise should be sufficiently happy with the ppc code base for purposes of exploring the kernel, or using the kernel as a base for driver development.
While having the option for building one's own kernel, how often does anyone really do it? In using Linux for over ten years, I think I compiled a custom kernel only once, and it was because I was having a sound card problem before the time of kernel loadable modules. With the current design of xnu (and most UNIX-like kernels for that matter), the need to rebuild a kernel is minimal if not completely without necessity.
by Tom Bradford — May 19
At that point, it only matters that you don't have the i386 code in very odd scenarios, but otherwise should be sufficiently happy with the ppc code base for purposes of exploring the kernel, or using the kernel as a base for driver development.
While having the option for building one's own kernel, how often does anyone really do it? In using Linux for over ten years, I think I compiled a custom kernel only once, and it was because I was having a sound card problem before the time of kernel loadable modules. With the current design of xnu (and most UNIX-like kernels for that matter), the need to rebuild a kernel is minimal if not completely without necessity.