Well, that would make total sense if apple's view on it didn't seem to shift around so much. System Preferences and Photo Booth only have one window. When you close that window, there is no other possible window to have open. Disk Utility closes if you close the main window. However, there is another window that can be opened, the disk utility log window. If that window is open, the application does not close, and continues running like most mac applications. If that behavior is followed, is there much sense in having iCal or Address Book continue running if there main window is closed? Or most any application for that matter? Maybe only certain apps should remain running when all of their windows are closed, such as Safari and Finder. Applications where there is not one defined main window, but where any window might be the primary focus at any given time? Just a thought.