from today's Japan Times:
"Boeing chose lithium-ion batteries for the 787, which uses five times more electricity than similar jets, because they hold more energy and can be quickly recharged, Mike Sinnett, chief 787 project engineer, said. In a worst-case scenario in which the batteries do burn, they are designed to do so in a way that doesn't threaten the aircraft, Sinnett said.
If the jet is airborne, smoke is supposed to be vented out of the compartment so that it doesn't reach the cabin, he said, and all of the battery cells can ignite without harming the plane's ability to stay aloft."
So that must be a heck of a fire shield...