Autonomous vehicles have been legal within the state of California for a while. However, the California DMV just set down some new testing regulations that are now in effect. The new regulations are fairly stringent.
Autonomous vehicles now require a person to be seated in the driver's side. Drivers must be employed by the manufacturer and must have a special testing permit, which can only be acquired after going through a training program. Drivers must remain attentive during testing periods and are required to report to the DMV any time they manually override the car's controls or get into an accident.
Testing is also not going to come cheap for manufacturers, as the DMV is requiring automated vehicles to have at least $5 million dollars worth of liability insurance.
The full text of regulations can be found on the California DMV website.