Log in to the iPhone Dev Center and take advantage of all the development resources available to you, including the new version of the iPhone SDK.
This eighth beta release of the iPhone SDK includes a complete set of tools, compilers, frameworks, and documentation for creating iPhone OS applications. These tools include the Xcode IDE, and the Instruments analysis tool, among many others. With this software you can develop applications that run on iPhone and iPod touch using the included iPhone Simulator.
Beta release 8 is compatible with the final iPhone OS 2.0 release and must be used to build and sign any iPhone OS application to be submitted to the App Store. As a reminder, you must be a member of the iPhone Developer Program to submit apps.
More info here.