Battery on white MacBook shuts off abruptly after 30% charge is left
I had the exact same problem (twice!) and had my battery replaced under warranty - I seem to recall there was a battery replacement program for this issue (don't quote me) but even if there was I suppose that would have finished now. 

I'd say you might have a chance if you called and explained the issue or went into an Apple store depending on what kind of person you run in to.

There was also a firmware update that was meant to help solve this issue, but it didn't help me and I suppose you'd have gotten that through software update long ago.

Long story short: It's not that the battery isn't holding enough charge, and not that it's had too many cycles - evidently they just had an issue with their battery production, and your machine has taken this long to be affected by it. Hopefully you get someone nice from Apple, but otherwise a trip to eBay or somewhere else on the internet should be able to get you a new battery for relatively cheap. 

Good luck, hope it works out for you.