Beside the update version of Mac OS X Lion Developer Preview 2, Apple has also released Xcode 4.1 Developer Preview 4 to Mac OS X developers. It is required to build apps for OS X Lion. Xcode 4.1 Developer Preview 4 is available now in the Mac Dev Center.
This is a pre-release version of Xcode 4.1 for both Mac and iOS development. This release requires Mac OS X Lion Developer Preview 2 Update 2 and includes iOS SDK 4.3. Continue to use Xcode 3.2.5 or Xcode 4 on a Snow Leopard partition if you plan to submit Mac or iOS apps to the App Store.
This release adds IB support for NSPopover and NSWindow animations and behaviors among other enhancements.
You can download the update from the Mac Developer Center.