Apple has announced a worldwide Battery Exchange Program for its 15" MacBook Pro. According to Apple, the affected batteries do not pose a safety risk, but rather "do not meet [Apple's] high standards for battery performance... You may continue to use your current battery until a replacement arrives." Arstechnica speculates that the cause of the exchange may be due to random system shutdowns that some users have been experiencing with faulty batteries.
The affected batteries have model number A1175 and a 12-digit serial number that ends with U7SA, U7SB or U7SC.