Here's a brief FAQ:
What's wrong with the batteries?
The recalled lithium-ion packs can overheat and rupture. They pose fire and burn hazards to consumers.
Have people been hurt?
Yes. Since May 2010, HP has received 40 additional reports of laptop batteries that overheated and ruptured. These incidents resulted in 7 burn injuries, one smoke inhalation injury, and 36 cases of property damage.