read this elsewhere:

NVIDIA has officially given some information on the lack of support for the Yorkfield on its existing nForce 680i motherboards through a document in question/answer format.

To sum things up, NVIDIA confirms as we indicated yesterday that nForce 680i SLI motherboards are incompatible with Intel’s 45nm quad core, the Yorkfield, but they did say it will function with the dual core. This is a hardware based problem because a supplementary circuit is required on nForce 6x0 and nForce 7x0 motherboards to support the CPU. Moreover, support via a simple bios update is not foreseeable.

NVIDIA specifies that the circuit was not required with pre-production Yorkfields and that it is not possible to modify current motherboards to add this support. Of course, nForce 7x0 motherboards will (we hope) all be equipped with this circuit and we can assume that the delay in its release is related to this modification.