As many people already answered, iPhone SDK is available only for OS X, and I believe Apple will never release it for Windows. But there are several alternative environments/frameworks that allow you to develop iOS applications, even package and submit to AppStore using windows machine as well as MAC. Here are most popular and relatively better options.
PhoneGap, allow to create web-based apps, using HTML/CSS/JavaScript
Xamarin, cross-platform apps in C#
Adobe AIR, air applications with Flash / ActionScript
Unity3D, cross-platform game engine
Note: Unity requires Xcode, and therefore OS X to build iOS projects.