Mobile websites use adaptive design. This means that the page width adjusts to the width of the screen of the visitors device.
They also need finger size buttons for navigation, special inputs for forms, finger swiping slideshows etc.
iWeb, like all template based eidtors, is not capable of creating this type of website.
Here are some example designs....
http://www.ezmacwebdesign.com/mobile-website-design/showcase.html
All of these could be created using an application like Flux which, by the way, is being offered at a very reasonable $39.99 by Smith Micro at the moment.