Motherboard manufacturers have several choices implementing SLI support on their X58 models.
They can license SLI from NVIDIA and certify the board in their labs or use separate nForce 200
bridge chip/chips. Now Asus has introduced P6T6 WS Revolution:
This board features 6 PCI Express x16 slots.. three of them (blue ones) are x16 for 3-Way SLI. This
is made possible with two NVIDIA nForce 200 bridge chips on board which offer enough PCIe lanes.
For example Rampage Extreme will only offer 16x/8x/8x configration for 3-Way SLI.