Nvidia has mentioned in the past that they are working on a 40nm manufacturing process for their GPUs, a natural upgrade from the existing 55nm parts. They haven't given many details beyond that, though now it is rumored that the first 40nm parts will begin making their way out of factory doors during the middle of next year.
Whether or not they will beat AMD to that mark isn't clear. AMD's RV870 chip, which will be a 40nm part, is also scheduled to come out around the same time - roughly Q2 of 2009. Cards from both companies will see the obvious reduction in power consumption, so the real question is which company will produce the faster card. At the moment, it seems like AMD will - because Nvidia is apparently going to be using their existing G92 architecture to manufacture 40nm parts.
Given that the existing Radeon parts are doing well to beat the G92, things are not looking up for Nvidia.