Yeah but, on average, what costs the most? A graphics card worth SLi'ing or a chip or three on the motherboard? I know they want to sell chipsets, but they'd sell so much more *product* if they allowed SLi on other chipsets. Just look at Crossfire, the AMD chipsets aren't all that popular, but Crossfire has a larger potential userbase because it's possible on Intel chipsets.
I know at the moment the nVidia chipsets' main selling point seems to be the ability to use SLi, but I think they should just forget chipsets for this generation. Allow SLi on other chipsets then use the extra money to invest in making the nforce 790/880 much much better and see if they can pull back some customers with a much more stable, compatible and overclockable chipset with things like ESA and hybrid SLi etc.