I do not believe so, Nvidia only designs and produces the chipset that is used by the other manufacturers in creating the actual card. And as far as I know they do not manufacture any of their own cards.
Yes you are right, I've never ever seen an nvidia-manufactured card in my life. But if I look at ATI their card only use stock speed and design and their look is pretty basic. While other manufacturers like Sapphire, XFX, MSI, will factory overclock them, use better cooling solution, and have better look. I guess nvidia would probably do the same if they'd start to build their own card, they would let their partners design the best cards not to steal any market from them.