What impact does leaving Pandora paused in the background have?
Almost none. There is no impact at all on battery life or CPU performance while Pandora is paused. Similarly, although a small amount of memory is used to keep Pandora in the background, iOS4 can intelligently manage this to prevent any performance impact. IMPORTANT NOTE: For best performance, and to prevent unnecessary data streaming, be sure to pause Pandora– instead of turning down the audio volume– when you are done listening for a while.
