Interface Builder is integrated into Xcode 4.
All the window or views in a xib file will show in the design canvas not in a separate window any more.
There is a media library in the interface builder. You can preview and add a picture to your UI directly.
Select a control, press Option key, and move your mouse, you will see some useful annotations.
In Identity Inspector, there is a small arrow at the right side of the class. You can click it to browse the definition of this class.
When you select a control, the Quick Help will show some useful info including sample code.
Ctrl-drag or right-button drag to make connection, insert IBOutlet, IBAction.
Xcode Assistant content: Top-level objects, custom classes, classes with connections.
It is convenient to use Interface Builder in Xcode 4, though there is no more new about Interface Builder.
WWDC2010 Session315 Using Interface Builder in Xcode 4
Author: Kevin Cathey - Interface Builder Engineer