From The Dock, at developer.apple.com:
An application should always present a window when its Dock icon is clicked. This behavior compensates for the all-too-familiar case where a user closes a document window and thinks she has quit the application. The next time the user wants to use the application, indicated by a click on its Dock icon, a new window opens so that the user is absolutely sure that the application is active.
Does anyone else find this feature slightly annoying? Maybe I'm just used to Mac OS 9, but I don't appreciate applications opening a window without my say so. Maybe I don't want a huge browser window to pop up every time I switch to Safari. Maybe I switched to TextEdit for the open dialog, not a new blank document. This behavior is somehow intuitive? "All-too-familiar case," indeed.