Ha! You're wrong. It's all about the iPhone.
See, the iPhone is designed to let you run custom applications on it. The catch is that they have to be web apps, and Apple isn't providing much in the way of developer tools. So how will you know if a web app will work on the iPhone?
If it works in Safari.
(I learned something today!)
If the applications are "pure" web applications, then you'll be able to test them out on any browser, in particular Firefox (since IE is buggy as hell to start with). No doubt it's going to be the same setup as with Tiger's Dashboard widgets which you can at least start developing and testing through any regular web browser.
Also, Apple will no doubt want anyone who develops applications for an iPhone to do so on a Mac through
XCode and through the upcoming
DashCode applications.
They'll no doubt release "iPhoneCode" or something like that as either an extension or a standalone application at some point. In which case since they'll build it on WebKit (the engine behind Safari, based on the original KHTML) that's where their testing of Safari on Windows could pay off if they decide to port the app to Windows.