For laptop users, Snow Leopard has a new feature that reports the condition of you battery to you. To see this in action, hold down the Option key and then click on your battery indicator in the menu bar. It'll show an additional line at the top of the menu that lets you know if your battery is okay or not:



For reference, here are the possible results for the condition of the battery:

Normal: The battery is functioning normally.

Replace Soon: The battery is functioning normally but holds less charge than it did when it was new.

Replace Now: The battery is functioning normally but holds significantly less charge than it did when it was new. You can continue to use the battery until you replace it without harming your computer.

Service Battery: The battery isn’t functioning normally, and you may or may not notice a change in its behavior or the amount of charge it holds. Have your computer checked by an Apple Authorized Service Provider (AASP). You can continue to use your battery before it’s checked without harming your computer.