Apple Wednesday afternoon released the sixth beta version of the iPhone SDK.
According to the notes, beta 6 requires OS X 10.5.3, and will not function correctly on previous versions of OS X. The SDK is a whopping 1.19 GB download (which I'm currently pulling down at 200 kb/s ).
From the readme:
This sixth 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.