Elon Musk said on the earnings call today that if you buy a car with the 300 mile pack you will have a longer warranty than the other packs. A Tesla employee explained this to me at the October 1 event. Because the average person drives say 40 miles per day. The 300 mile pack is only cycling 40/300 = 13% whereas the 160 mile pack has to cycle 40/160 = 25% per day. So the 300 mile battery pack will last longer (ceteris paribus) and the warranty will be consequently longer.