This is a really interesting article on battery replacement..
Every time I buy a new Mac laptop, I question whether I should purchase AppleCare to extend the warranty from one year to three years. My MacBook Pro cost $2,800 (with tax and shipping) in November 2006, so laying out another $300 for AppleCare - well, frankly, it hurt. (For more on the purchase, and how it stacked up to previous PowerBooks I've owned, see "More Bang, Less Bucks for my MacBook Pro" 2006-11-20.) However, I've found that almost every laptop I've owned has needed some sort of after-warranty work done, so I've ordered AppleCare for every one. [From Making AppleCare Worthwhile: MacBook Pro Battery Replacement]