This is a pre-release version of Xcode 4.1 for both Mac and iOS development. This release requires Mac OS X Lion and includes iOS SDK 4.2. Continue to use Xcode 3.2.5 on a Snow Leopard partition if you plan to submit Mac or iOS apps to the App Store.
You can check out the new Xcode 4.1 user interface and feature list at the link below. Developers can download the release from developer.apple.com/mac. Interestingly, Apple is making the release available via a Mac App Store Redeem Code.