The answer is: Yes you can absolutely do iPhone and iPad website development on a Windows PC.
However, you really should/must test the result on an actual iPhone/iPod Touch/iPad. Especially if you are integrating in any way with special device features like the dialing feature of the phone. (Yes you can have phone numbers in a webpage trigger dialing when you tap on them.)
However, you can do the bulk of the development on Windows, testing the WebApp in Safari or Chrome, which are the most fully compliant HTML5 WebKit based browsers out there.
Also highly recommend using an HTML5 touch framework like jQuery Mobile or Sencha Touch. This will go a long way to ensuring that your WebApp is optimized for the screen size and touch gestures of the mobile devices.
Remember that you can't deploy a pure WebApp to the app store, only download it from a website. You'll need a native wrapper like PhoneGap for that. And to compile a PhoneGap wrapped WebApp you'll need XCode on a Mac.
But there's a lot of power in adding your WebApp to the home screen on iOS. No native code involved and you get a full screen webapp with a home screen icon, loading image and no browser toolbars. Highly recommended.