No official SDK but you can try DragonFire SDK or one of the other alternatives that transcode another language into an iPhone application such as Adobe Flash CS5
Only Macintosh computers because the Software Development Kit is just a set of APIs, Libraries, and Runtimes that extend XCode (the Macintosh Developer Tools).
There are a ton of frameworks and such that only exist on the Mac OS X development layer that the iPhone SDK needs in order to compile and run.
And also, the language that the SDK uses, (Apple uses Objective-C) doesn't exist in Windows, which is why Apple won't go through the year or more's work to port XCode just for Windows users (especially since anyone using Windows wouldn't know the Objective-C language, so literally 97% of those people would try the SDK a couple times, then give up...not worth a whole year's work of porting).
Source(s):
iPhone/iPod Touch developer
Ok there is something on google code I found called winchain. Google it. It has a fake Mac filesystem. What u do is Download the prebuilt toolchain, unrar it winrar or sevenzip then get winchain builder and built the toolchain. It's very complicated and the directions are on the google code page. I hope this helps.