Xcode 4.3 is now available as a standalone app in the Mac App Store. This version replaces the Install Xcode package and adds support for delta updates. Xcode 4.3 also has a new "Downloads" preference pane that lets you install optional components, such as command line tools and previous iOS Simulators.
Xcode 4.3 in the Mac App Store includes the OS X 10.7.3 SDK and iOS 5 SDK. If you are developing iOS 5.1 apps, you should continue to use Xcode 4.3 Developer Preview with iOS 5.1 SDK beta, available in the iOS Dev Center.