Apple today announced that developers would be using the same APIs and basically the same SDK that Apple uses to build iPhone apps. Developers will be using Xcode to build the applications and the updated version of Xcode should be available today.There is also an iPhone simulator which works as a full on iPhone emulator for developers to test out applications without having to install it on their device. Apple does claim however that it is very easy to connect your iPhone to your Mac and run the application on your iPhone for live testing.

Engadget

Update: Anyone can become an iPhone developer, it will cost $99. You’ll get the beta version of the iPhone software update (link) and you’ll get the new version of Xcode (link).