If you have dedicated a section of your app for an iAd, it's important to incorporate a banner view delegate that will handle common events, including errors. Having the proper delegate to handle errors maintains the visual experience customers have with ads in your app by preventing a blank banner from being displayed when ads are not available. In addition, if your app displays a blank banner when advertisements are not being displayed, your app will be returned to you by the App Review Team for modification before it can be posted to the App Store.
Read the Working with Banner Views section of the iAd Programming Guide for details on using the bannerView:didFailToReceiveAdWithError: method so that your banner view responds correctly when advertisements are not available. Registered Apple Developers can also watch the WWDC 2010 session video Integrating with iAd, which covers ad banner events in the Application Frameworks section of the presentation.