Yeah we don't have it yet, but I guess in time it will come like Audi' "connect" which is only just now available for order on the A3 some 6 months after launch.
Essentially, car net is a way of receiving information via the Internet in the car on the move. This can be used for example, googlemap overlays for the sat Nav, real time traffic jam reporting and displaying, live POI searches and picture searches etc. as well as generating a wifi network in the car for passengers to log onto for example their iPads.
Trouble is, car net is only available with a certain hardware combination. Firstly a prerequisite is Discover Nav Pro (the other Navs will not work). Secondly mobile phone premium. Using this system the car can connect in one of 2 ways. Firstly, if the phone connected to the inductive phone plate in the armrest is rSAP compatible, then it will use this phone and sim to download the relevant information. Additionally, the Nav hardware installed in the glovebox (that has the 2 SD card slots), in addition would have a sim slot. This sim slot will take a standard phone sim with a data and talk allowance, and use this instead of the actual phone.
The €100 extra in the German price list for Car net, as I understand it, is for the modified glovebox unit with the Sim slot, (and its this that currently isn't available in the UK), but as said, you also need the premium phone pack as well and Navi pro.