Many companies and developers are crafting iPhone-specific interfaces for their websites. Why do that when the iPhone perfectly renders their existing page? The aforementioned slowness of the Edge network is one reason. These iPhone specific portals are lightweight and fast, even without Wifi. Many of them also work very hard to replicate the glossy icon based interface on the iPhone, making the website feel like a native application.
iPhone users are early adopters, technically proficient and skew to a younger demographic. If that's a sect you'd like to market to, an iPhone specific website may be a good fit for you. If not, don't worry. iPhone users can see your website just fine.
We've been loving our new iPhone. It's an amazing device, fully realized as a first-generation product.