I will admit that the browser on today’s iPhones and iPod Touches are impressive. They both do an amazing job displaying Web sites as the designer intended. However, some sites still work much better than others. Recently I’ve been pointed to to a neat little bit of HTML that can improve how your site is displayed on these platforms. Just add this line to the head of your document:
<meta name="viewport" content="width=320">
There are other attributes available including
maximum-scale. More details can be found on the iPhoneWebDev Google Group.
Source: iPhone Geek