"Apple has initiated a worldwide battery exchange program for certain rechargeable batteries that were sold for use with 15-inch MacBook Pro computer systems from February 2006 through May 2006.
We recently discovered that some 15-inch MacBook Pro batteries supplied to Apple do not meet our high standards for battery performance. To give our users the best experience possible, we will replace these batteries for customers free of charge.
The affected batteries have model number A1175 and a 12-digit serial number that ends with U7SA, U7SB or U7SC. To view the model and serial numbers located on the bottom of the battery, you must remove the battery from the computer. The battery serial number is located above the barcode."
It seems that this exchange is because of performance issues rather than safety, but it's still worth taking advantage of it.
For more details visit the Apple support site.