There is no built-in way to dim the screen unless you're presenting a view controller, at which point iOS dims the background view controller for you.
Instead, if you want to dim stuff you need to do it yourself: create a full-screen
UIView with a translucent background color (I find 66% black works best) then set its alpha to be 0. When you want things to dim, set the alpha to be 1.
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Available from iOS 2.0
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