VR-Zone has posted some details on how NVIDIA's Quad SLI works. They heard from an interview with George Myers, NVIDIA's desktop chipset manager, that the Quad SLI solution is different from the SLI technology on the Gigabyte 3D1 and ASUS Extreme DUAL graphics cards:
Each of the Dual 7800GTX 512 card is specially made and has an additional chip onboard to control the data transfer to both the GPUs for calculation. Quad SLI can be used on nForce 4 SLI x16 AMD as well as Intel Edition.
The reason why Quad SLI uses 1-3 2-4 MIO connection method is to reduce the Northbridge and Southbridge HTT bandwidth usage as the calculated data can be transferred from the Dual 7800GTX 512MB card on the Northbridge through the MIO to the Dual 7800GTX 512MB card on the Southbridge.