If you are planning on getting your first iPhone app, now you can create a mockup UI on Windows with MobiOne. The software brings some cool features like Drag-n-Drop UI layout, Rich UI component palette, Smart guides and snap alignment, Export mockups as image.
A future version is already planned to support UI development with HTML generation for iPhone and other popular devices such as Android, RIM Blackberry and Nokia.
You can find more info and download the software for free here.