Take the question seriously... I don't think Apple can attract people to the Mac platform only providing an app like Safari. Building Safari on windows is probably a relatively big effort, even if the GUI toolkit was already here for iTunes. But Windows is still the dominant platform. And Apple has Dashboard widgets, its extensibility mechanism for the iPhone... So I think Safari on Windows is at least here to let iPhone users know that yes, they will be able to have on Windows the side-apps they install on their iPhone, and allow developers using only Windows to write and test side-apps for iPhone. On another note, iPhone expected sales should immediately promote the device to a nice rank in the most-important-cell-phones list. This implies that more cellphone manufacturers are going to look at the rendering engine of the browser in the iPhone. Having it available for all platforms is then necessary, because it shows the ubiquity of the solution and allows embedders to think of open solutions.

I might be totally wrong, but at least this is plausible.