There is a nice review in Nature Reviews Neuroscience on the mechanism that causes asymmetry in the brain. One of the theories discussed is that molecules produced in the embryonic brain that induce cell specialization are distributed asymmetrically in the hemispheres, which starts a chain reaction leading to hemispheric specialization. Competing theories have suggested fetal position, or development that occurs postpartum.
Handedness is interesting to neuroeconomists and others because imaging studies frequently exclude left-handed individuals because activation patterns in left-handed individuals are inconsistent even among lefties, and don't correlate well with data obtained from right-handed individuals.
Almost of equal interest in this review is the fact that hemispheric specialization may be influenced by the "Sonic Hedgehog" gene. I'm glad that (a) I have a Sonic Hedgehog gene and (b) such a gene exists in the first place.