Although I really like the idea of a single window for XCode and IB, I don't like the idea not being able to use my 2 monitors, having 2 open windows, 2 XCode projects where I can view changes, compare and update each project according to my needs. You see I work with my custom made .dylibs all the time and being able to see the files of a framework on one screen and the files of an end-user application in the other is crucial for me.
It's one reason I don't like Visual Studio. And It's one reason I don't like Windows in general. Apple has always adopted a Single-Application-Many-Windows approach. Windows has adopted a Single window Single application approach.
There is no point in making a flexible program less flexible. For me, XCode needs many improvements in the UI, but doesn't need to be the next Visual Studio. I hope they give the opportunity to dock and undock windows from the main window.