nVidia Quad SLI: a 4 GPU-based SLI:

While the SLI technology allows users to get GPU performance boosted by using 2 graphic cards in the same computer, nVidia has started to promote its new technology flagship: Quad SLI. This is a new step towards the maximal performance of graphic cards, as it is meant by its name; you simply get the power of 4 GPU while having only one output.
To achieve it, nVidia is using dual GPU-based graphic cards.

Clybic has tested this new solution:
http://www.clubic.com
Of course it is a beast, but as mention by the author of this test (in French), it is not of real interest except to play GPU-demanding games in max resolution (2560x1600) on a 30".
In addition, such graphic cards are rather heavy (2 GPU + huge cooling system), not recommended for all PCIe slots.
To conclude, it is a marketing-oriented technology, since the price is this system is simply too high; but it makes use of an old technology originally developed by 3Dfx, before being acquired by nVidia.