A single two-dimensional Dirac fermion state has been recently observed on the surface of the topological insulator Bi2Te3 by angle-resolved photoemission spectroscopy. We study the surface band structure using k x p theory and find an unconventional hexagonal warping term, which is the counterpart of cubic Dresselhaus spin-orbit coupling in rhombohedral structures. We show that this hexagonal warping term naturally explains the observed hexagonal snowflake Fermi surface. The strength of hexagonal warping is characterized by a single parameter, which is extracted from the size of the Fermi surface. We predict a number of testable signatures of hexagonal warping in spectroscopy experiments on Bi2Te3. We also explore the possibility of a spin-density wave due to strong nesting of the Fermi surface.