Think of 2 compact V6s smashed together. Volkswagen did it with their top-of-the-line Phaeton luxury sedan, as well as for a new super-car concept they were coming up with. Which would make great cars the only problem being they had a VW badge. Same thing with the Phaeton ....it is a great sedan, but it is hard to plop down enough money to buy a 7-series or an S-class on something with a VW badge. The ironic thing is that VW owns Bentley, Porsche and Audi, and that VW by itself is a really serious and very competent manufacturer.
Anyways, a W-12 is basically 2 compact V6s put together. Maybe Spktyr can give a better answer (hint hint Ping for you Spktyr LOL).
Here is a pic of the VW supercar concept.
Would be a great car to compete with with Ferrari 430 and the Lamborghini Gallardo (and even the Murcielago). But again note the badge issue. It might be prudent for them to consider badging it differently ....maybe as an 'uber' Audi (VW also owns Audi)to avoid some of the issues they got with the Phaeton. Oh, and the upcoming Audi A8L with have a W12 if certain things work out.