Apple has just posted their iPhone Web development guidelines. (Thanks John.) As expected, the iPhone integration seems limited to a few “special links”.
Mailto: works as expected, opening the built-in mail application with a new compose-mail sheet. Google Maps linksare recognized by the iPhone and opened in the iPhone Maps client. The phone link (tel:) calls the specified number when clicked, e.g. <a href=”tel:1-408-555-5555″>1-408-555-5555</a>.
There are also some useful guidelines to be found about formatting media for smooth iPhone playback, a list of iPhone gestures (double-tap, touch-and-hold, drag, flick, pinch-open, pinch-close), a discussion of the iPhone viewport and more.