Apple provides a developer's guide for the iPhone.

Developers were surprised to hear that Apple wouldn't be releasing a software development kit (SDK) to write software for the iPhone. Instead, they said, developers could write iPhone apps using... Safari. In other words, iPhone apps would be Web 2.0 applications that use the Safari browser as their launching point.

Apple just released a developer's guide for building iPhone applications. Available through the Apple Developer Connection web site, the pages describe WebKit, Apple's open source browser engine behind Safari, and how Safari interacts with other applications and functions on the iPhone.

Scott Gilbertson writes in the Wired blog Monkey Bites that enabling Web 2.0 apps for the iPhone is simply "a matter of dumbing down your app for Safari" on the iPhone. The developer documentation describes the limitations associated with Safari on the iPhone, but they also describe how it connects to iPhone functions. In addition to guidelines for optimizing the way an application works and displays on the iPhone, there are explanations of iPhone-specific user gestures (flicking, double tapping, pinching, touch-and-hold, and dragging), the "tel" protocol (<a href="tel:212-555-1212">Call</a>) for making calls, specifying iPhone-specific CSS stylesheets, and much more. There are lists of what Safari on the iPhone does and doesn't support (although the fact that you cannot download files to the iPhone is not explicitly mentioned). Gilbertson is probably right in entitling his blog post "How To Cripple Your Apps for the iPhone", but given the limitations of virtually all portable devices, this information is a great thing to provide to developers.