Rogue Amoeba has released an update to Airfoil Speakers Touch adding support for background audio on multitasking capable iOS 4 devices. Airfoil Speakers Touch allows users to stream audio to their iOS device from a Mac or PC on the same Wi-Fi network, effectively allowing the iPhone or iPod touch to act as a set of remote headphones. With the latest update users of the iPhone 4, iPhone 3GS and third-generation iPod touch can now keep Airfoil Speakers…