I started a new project in Xcode 4 using the TabBar template.
I noticed that there was no MainWindow.xib
file in the project.
MainWindow.xib was kind of superfluous to begin with, keeping in mind that it doesn't really contain any "visible" components (thinking of the window as an invisible container) and that usually you don't need to modify it. (That said, I'm only a beginner in iOS development myself, so I would appreciate my observation being corrected if I'm wrong).
MainWindow.xib's role in the beta version has been replaced by code: compare the auto-generated (-)application:didFinishLaunchingWithOptions: of the same template in the current and beta version of Xcode, and you should be able to follow what's happening.