From reading his webcomic, it seems that he is an expert in not only programming:
http://xkcd.com/163/and computer science:
http://xkcd.com/399/but also physics:
http://xkcd.com/505/*
http://xkcd.com/849/astrophysics:
http://what-if.xkcd.com/129/rocket science:
http://what-if.xkcd.com/85/advanced mathematics:
http://xkcd.com/1310/statistics:
http://xkcd.com/696/and cryptography:
http://xkcd.com/936/*Note:
Graphs
The three diagrams in the "Physics, too. I worked out the kinks..." panel are, from left to right:
The Normal distribution of the Gaussian curve marking the points that represent a standard deviation of σ and 2σ. This is one of the fundamental building blocks of statistics. In quantum mechanics particles are viewed as inherently random, therefore the time at which a particle will decay, the position of a particle and its velocity are all calculated using similar curves. A deviation of at least σ occurs 50% of the time where a deviation of 2σ or more occurs about 5% of the time.
The Epitaph of Stevinus, an explanation of the mechanical advantage of using an inclined plane. The inclined plane is one of the six classical simple machines, one of the fundamental building blocks of mechanical and civil engineering.
The last graph is unknown. It may represent coupled pendulums, length contraction, or a hypothetical solution to something we haven't derived yet.
The graph that represents particle interaction is a Feynman Diagram. This shows the interaction of subatomic particles that collide and exchange some momentum via a photon. The slope of the middle line represents the distance moved and the time lost/gained during the interaction.
It seems that he is now writing full-time after a brief stint at NASA. A bit unfortunate I guess since he's no longer contributing to science but it seems to be what he enjoys most (plus his comics are pretty popular!).
William James Sidis was also very similar. He was a genius in mathematics and cosmology but later chose to focus on his hobby of collecting street car transfers instead.
