This is an issue for lots of sites now!

The answer depends on what you mean by mobile friendly. Some sites rethink the site entirely for mobile users, presenting modified content as well as design that will be more relevant and easier to use in a mobile context. Here’s a good reference on the topic of what really being “mobile friendly” means in that broad sense. And the author of that presentation, Luke W, has many other presentations and posts on the topic.

Others are coded in a way that reshapes and styles the interface elements to display legibly and usably in a new scale—whether a smaller browser window or a smaller mobile device. You can read about that here. Also more on mobile design and coding approaches at A List Apart.

On the simplest level, just for legibility, you want to make sure there’s nothing that can’t be displayed on the iPhone such as Flash. And you can leverage an existing platform, such as WordPress (as this site does), that has mobile-friendly display built in. WordPress enables the site to display nicely on iPhone, other mobile devices and even iPad.