Yakima now sells a roof rack that mounts on the Model S. It works only with cars equipped with the Panoramic roof option, which includes specific roof rack mounting points.
I suspect that any roof rack system designed to work with modern BMW roof mounting points would work on the S, as the mounting bolt holes look identical. These are located under little flip-up access doors on the roof on BMWs and the Model S.
The Yakima rack can be found here, but is a bit tricky to locate:
To find the rack, you must select "Tesla", "2012", and "Model S With Panoramic Sunroof". Do NOT select the 2013 model year, as the Yakima online selector is apparently not updated for 2013 and will not locate a fitment option.
I do not plan to purchase one, but found it when considering options to carry bicycles. We went with a rear hitch mounted rack instead (covered under a separate forum thread), but thought some might find this information useful for carrying longer things like skis, kayaks and tandem bikes.