From this announcement
In cooperation with the U.S. Consumer Product Safety Commission, on May 14, 2009. HP announced a worldwide voluntary recall and replacement program for battery packs used in certain HP notebook PCs. HP customers with a battery pack affected by this program will be eligible to receive a replacement battery pack for each verified and recalled battery pack at no cost.
The text goes on to offer ways to find out if you are affected and how to proceed - HP Notebook PC Battery Pack Replacement Program