From the Read Me file:
iPhone SDK for iPhone OS 2.2.1 is an update and bug fix release to the iPhone OS developer tools. This release includes new, optional GCC and LLVM compilers, updated project assistant, a simplified toolbar, and additional security enhancements.
The iPhone SDK includes the complete Xcode 3.1.2 developer tools for Mac OS X development as well as the SDK components for iPhone OS development.
We encourage developers to apply to the iPhone Developer Program for access to additional support resources, as well as provisioning resources to enable development directly on an iPhone or iPod touch. For more information visit:
Compatibility with Mac OS X Versions
iPhone SDK requires an Intel-based Mac running Mac OS X Leopard version 10.5.4 or later. Xcode 3.1.2 is also available separately for Mac-only development and is compatible with Intel and PowerPC Macs running Mac OS X Leopard 10.5 and later.
• SDK support for targeting non-Mac OS X platforms, including iPhone OS SDK.• Integrated SCM support now works with Subversion 1.5
• GCC 4.2 & LLVM GCC 4.2 optional compilers for use with Mac OS X 10.5 SDK
• Updated assistants to create new projects, targets, and source files
• Toolbar uses a single popup to choose platform, target, and debug/release