Math/stats + ultra lean code + plus understanding market micostructure. From the CS side some of the more cutting edge stuff I've heard of being worked on involves hacking graphics cards to get a speed edge and building data centers in the middle of the ocean for the same reason.

http://www.nvidia.com/object/computational_finance.html

http://io9.com/5784758/the-speed-of-light-could-turn-the-mid...