In a further world premiere, Mercedes-Benz will enable access to the voice-based intelligent companion Siri™ in the vehicle. This allows natural speech input, without the need for specific commands, to send messages, select music tracks and check the weather forecast or stock prices, as well as to make appointments. All the iPhone® integration functions can be used even with the entry-level Audio 20 system. [Source: Autoblog]
And there you have what is pretty much the most expensive and elaborate accessory for you iPhone. It will debut on the A model, with the others following later.