Apple has published a GM for Xcode 4.1 in the Mac Dev Center this afternoon in preparation for the launch of Lion (presumably later this week). You’ll find an updated Xcode 4.1 GM, Read Me, and Release Notes that are recommended for consumption as developers beginning building Lion compatible apps.
Xcode 4.1’s Interface Builder includes support for new Aqua controls such as NSPopover, Full screen support in the Xcode workspace (for full screen apps), and project modernization that can automatically fix errors caused by improper build settings.