Elon Musk and his team of engineers have taken some criticism in the beginning, and outsiders have speculated that the packs made from thousands of cells were a compromise. However, it my understanding that the Tesla cars have more than one full-voltage sub-pack to them (I could be wrong). If a cell in a string begins to get hot, the computer cuts that string out of the pack-grid, long before it can become an issue...and the car can still drive on with slightly less range to the pack.
A dealer who has a Tesla come back in because of a "check battery pack" code, can quickly identify which string of cells has the bad one. It is a quick and simple matter to swap in a new string, and send the customer on his way. I now think this is a pretty damn good way to do it.