Apple is now allowing Made For iPod- and Works With iPhone-certified developers to include automatic application download prompts when connecting app-supported accessories.

So once you install an iPhone/iPod Touch accessory it will prompt you to download the specific app required to enable that accessory features, this would open doors for a lot of developers to take advantage of this in developing hardware based apps for the iPhone, which would allow us to see more devices that is going to expand the phones features and capabilities.

Sources : AppleInsider , ilounge