Call Apple
My girlfriend's macbook (2.4ghz intel core duo) was working fine a few days ago. She said she had it plugged in to charge the battery overnight, but failed to do so. Now she has to have it plugged in for it to work (and if the magsafe adapter is accidentally knocked off, the computer shuts down right away.) From what she noticed, it didn't have a gradual decline in battery performance (battery not holding charge as long, etc.)
Luckily, she still had a few months left on her AppleCare, so I called in with the hopes of sending in the computer since it might be an internal rather than a battery issue. Apple support walked me through the system profiler and the guy asked if it stated "battery installed- yes or no (which it said yes)," how many cycle counts (over 200) and how much charge remained (zero) on the battery. They diagnosed it was a battery issue rather than an internal one. At this point, I thought I had to buy a new battery since I know batteries weren't covered in the warranty, BUT--- the battery had known issues. Apple offered to send me a new one, and all they asked in return was send the old one back!!!
The lesson of the story- it is worth a shot to call them and see if there is a sort-of recall on the battery, even if your computer is not under warranty anymore. FYI- batteries aren't covered under warranty.
My girlfriend is happy. She can use her beloved Macbook without having to be continuously plugged.
Good luck, guys.