I thought we could gather our knowledge and rumours of the forthcoming Pagani in this thread and discuss.

Unveiling date: March 2008
Supposed project codename: C9

Horacio said a few years ago that the next Pagani will have a more powerful Mercedes AMG engine, will be a 2 seater Coupe, better built and better quality than the Zonda, better performance and also lighter. Improved in everything pretty much. And not based on the Zonda at all.
Together with the launch of the new car Pagani will also open their new factory which will allow them to produce the car in larger numbers in order to please the American market. As this car will be fully legal in America. Also, it won't share a single peice with the Zonda.

Developement of the new car started in September 2006.

Nothing 100% though as we haven't seen anything yet.