iOS Requirements for web development for iPhone Mac/Windows?

You don't need anything special, but Safari for Windows will help. You can enable Debug mode and set the user agent to Mobile Safari and then resize the window's width to be the same as the iPhone.

Technically, all you need for web development in Windows is to open Notepad. There is no special plugin or toolkit for anything. iPhone webapps are no different from any other web development out there. Many people will use custom javascript and CSS libraries to give the webapps an iPhone look and feel though (like iUI). Webapp development is platform independent and you don't need any special tools. Just a text or HTML editor. And as pointed out, install Safari for Windows to see how it will render on an iPhone. Firefox and IE do not render things the same way.