There are two possible reasons I can see Apple doing this:
- The want to improve website compatibility with Safari - by having a Windows version hopefully more page designers will check compatibility and thus their core OSX users will get better support.
- They want to use this as their iPhone development platform - Safari will have a mode for running iPhone applications in a browser window the same size and behaviour as the iPhone (which will be like Dashboard applications?).
My gut instinct is that this started when the iTunes developers tried to see how easy it would be to port other apps. Apple must have a reasonable number of Windows savvy developers by now...