This information is from 2007 but every time I make a web page for the iPhone I have to go and search for it.
The iPhone Safari browser does a good job of presenting just about any web page on its relatively small display. In so doing, it assumes that whatever web page it is displaying was designed for a much bigger display. To design a web page targeted for the iPhone and not have the phone shrink the web page to oblivion one must therefore add a meta tag to the head section of the HTML file:
<meta name="viewport" content="width=320, user-scalable=yes">