I just received an email from the Apple iPhone Developer Program notifying me of a new iPhone SDK, this being the second beta version (build 9A2151).

The second beta version of the iPhone SDK is now available and includes Interface Builder, a powerful tool that allows you to visually build your interface and makes creating a UI as simple as drag and drop. We've also added new sample code and updated documentation to the rich set of resources available to you in the iPhone Dev Center. Log in to the iPhone Dev Center today and download the new version of the iPhone SDK.