Up until recently, Quad SLI has only been enabled with per-system drivers designed to let OEMs equip systems with 4 GPUs Now, nVidia has released a beta driver that unlocks this functionality on any SLI motherboard, allowing enthusiasts with a lot of money and a desire to do it themselves the ability to run a Quad SLI system. The driver isn't finished, and as such nVidia releases it with a warning:

“This driver is a Beta driver that enables Quad Nvidia SLI technology, allowing you to put two GeForce 7950 GX2 graphics cards in a single system with an Nvidia nForce SLI motherboard. This driver is strictly for end user Beta testing with two GeForce 7950 GX2 graphics cards only and is not intended for production,” a statement by Nvidia reads.
nVidia released along with the driver a qualifications sheet that listed what minimum hardware is required to use the driver and probably will be very reluctant to provide tech support to people doing it. After all, when dealing with that much power, nVidia is correct in assuming that most do not know how to use it properly. That being said, with things like 32x FSAA now in our reach, it's a nifty present.