While the idea of an Nvidia-branded motherboard might be quite novel, particularly after the stunning success that has been the NForce series chipsets, the company has denied that they have such a project in the works. Apparently, though they have the technical and manufacturing capacity to achieve such a feat, their primary goal is to reduce the total time to market and increase volume:

The aim of the scheme is to enable Nvidia and its partners to reduce time to market and increase volumes, explained the Nvidia spokesperson. Additionally, it is believed that the program will help optimize performance of Nvidia-based hardware.
It is harder to do that if you are supplying everything from the top down, and given the popularity of the NForce series it might cause a lot of aggravation among motherboard vendors should they have chosen to make their own.