Here's an interesting piece from The Washington Post about the causes of handedness, as well as its cultural and evolutionary implications.
Money quote: "Among the Eipo people in the Indonesian province of Irian Jaya, where the homicide rate is 3 per 1,000 people per year, 20 percent of people are left-handed. Among the Dioula people of Burkina Faso, where the homicide rate is 0.01, 3 percent are left-handed. This does not mean that lefties are more violent. It means that in violent societies, lefties may fare better."
And, just like sexuality, no one knows exactly what determines handedness, but the current consensus is that it's a combination of genetics, environment and culture.