There is a Sony that has one. I don't know the model, but I know a previous co-worker chose it when we upgraded laptops specifically for the decent graphics card with an 11" screen for compact carrying (I was still surprised how much fatter it was than a MacBook Air, but I think that is down to the VGA port).
If you are seriously interested, drop me a line or reply and I'll track him down to find out.
Does anyone know of a 13" or smaller notebook with a good discrete GPU? The only one I know of is the Sager NP6110 [1] with a high end Nvdia GT650M.
If Sager can do it, why aren't there a lot more of these?
[1]: http://www.xoticpc.com/sager-np6110-clevo-w110er-p-4343.html