Apple's also making or preparing to make numerous under-the-hood type changes in Tiger. Some of these changes won't actually happen until 10.5 is released, but the preliminary steps towards making these changes are happening in Tiger. The changes Apple's making to the inner parts of the OS won't affect us end-users at all, and have, for the most part, only a small effect on developers (one change in particular, the deprecation of QuickDraw, has a HUGE effect on anyone still using it). The purpose of most of these changes is either to more clearly document parts of the operating system accessible to developers, improve performance, or both.