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I'm working on a music visualizer for the iPhone/iPad, under iOS 3 you could double-tap the home button and get iPod controls. With the latest version 4.1-4.2, these controls are now grayed out when the home button is pressed. I found a similar complaint at http://openradar.appspot.com/8696944, although there wasn't a solution.
I have the base sound category set to kAudioSessionCategory_PlayAndRecord, with kAudioSessionProperty_OverrideCategoryMixWithOthers set to true. (Just to add more fun to the problem I'm using OpenAL for some sound effects.)
I have tried setting the category back to ambient when the application goes into the background. but either it happens too late or it's not sufficient.