According to iLounge, Apple has begun to allow Made For iPod and Works With iPhone-certified developers to include automatic application download prompts. If the devices are connected to the Internet via Wifi or WWAN, the user can just plug in the accessory, then download the necessary software to get the devices to work.
A similar feature works for printers in Snow Leopard (and numerous other desktop OS functions). Plug the printer in and the OS goes out to the Internet to get the software it needs to work. Now this type of functionality has been brought to iPhone.
This functionality was introduced in iPhone OS 3.0 but this is the first time iLounge has seen it in action. They go thru the steps of installation of the Griffin iTrip Controller in screenshots below.