From the Feb 19, 2014 letter to shareholders:
"To further enhance the driver experience, new Model S customers will now receive free data connectivity and Internet radio for four years. As an added benefit to our existing Model S customers, the free four year period starts on January 1, 2014. To be fair to all, in rare cases a customer may be charged for extreme data use."
http://files.shareholder.com/downloads/ABEA-4CW8X0/3227824183x0x727013/9885dd26-2e82-4052-b171-3685fd8150b3/Q4'13%20Shareholder%20Letter.pdf