$10K was mentioned earlier in this thread. I don't remember the price myself.
The battery replacement program started at 15k and they lowered the price later to 12k which is what I paid in 2011, but it's pro-rated. If you replace it after 5 yrs you have to pay an additional 2k. At 8 yrs you pay nothing more. If you can wait until year 10, they give you 2k back.
The battery warranty, which is different, does not guarantee against capacity loss unless it is caused by failure of one of the components. In theory if you can start the car, and drive out of your driveway before running out, it's still a good battery. Of course it wouldn't do that unless there was something else wrong with it.