Advantages, problems, examples of adding another UIWindow to an iOS app?

Starting with Rob's answer I played around a bit and would like to write down some notes for others trying to get information on this topic:

• It is not a problem at all to add another UIWindow. Just create one and makeKeyAndVisible. Done.
• Remove it by making another window visible, then release the one you don't need anymore.
• The window that is "key" receives all the keyboard input.
• UIWindow covers everything, even modals, popovers, etc. Brilliant!
• UIWindow is always portrait implicitly. It does no rotate. You'll have to add a controller as the new window's root controller and let that handle rotation. (Just like the main window)
• The window's level determines how "high" it gets displayed. Set it to UIWindowLevelStatusBar to have it cover everything. Set its hidden property to NO.
• A 2nd UIWindow can be used to bring views on the screen that float on top of everything. Without creating a dummy controller just to embed that in a UIPopoverController.
• It can be especially useful on iPhone where there is no popover controller but where you might want to mimic something like it.
• And yes, it solved of course my problem: if the app resigns activation, add a cover window over whatever is currently shown to prevent iOS from taking a screenshot of your app's current content.

A UIWindow can float above other UI elements like the system keyboard.

To address your last paragraph: Make a UIWindow with the same frame as your main window. Set its windowLevel property to UIWindowLevelStatusBar. Set its hidden property to NO.