The thing on his shirt's the Mandelbrot Set, a fractal generated by iterating the function f(z)=z^2+C with f(0)=0 and C being the co-ordinate in the complex plane and the color representing the number of iterations needed to determine that the iteration escapes to infinity; black means it doesn't.

I drew it from memory and i think it came out rather nicely. XD http://commons.wikimedia.org/wiki/File:Demm_2000_Mandelbrot_set.jpg