Why is MacOS X required?

I know that a MacOS X machine is required for developing iPhone apps. What I would like to know is the reason for that. Is it just because a version of the iPhone SDK for developing on Windows PCs is not available yet but could be written? Or is there a technical reason that makes it impossible to develop iPhone apps on a Windows machine? i.e. Why can I not code & compile on a Windows PC, and then test...