The way SLI works is that there is code in the driver that looks for what motherboard it is plugged into. If it recognizes the that the board is a Nvidia chipset it unlocks and makes SLI avialable.
As for Intel motherboards or Intel Chipset motherboards, Intel did not want to pay the licensing fee that Nvidia wanted for them to put support into their drivers. Unfortunately we could all be happily running with SLI on our Intel boards if Intel had been smart enough to include SLI in the cross licensing that allowed Nvidia to use Intel's FSB and come out with their Nforce chipset that supports Intel processors.