Google Mac blog started a new series of posts that helps in learning “the inner workings of Mac OS X and how Apple achieves interesting effects”. You need to know how to read assembly, either PowerPC (if you have an older Mac) or x86 (if you have anything recent). While there are good resources available to learn about reading PowerPC assembly for exploration, there are fewer about x86. Despite the present and future of the Mac being x86, it seems like people have lots of anxiety about having to work with it.
More info: Google Mac blog