I had a similar problem. I had a 10 month old MacBook Pro. The battery was down to 40% health (according to Coconut Battery) with 80 cycles. It would only last 30 minutes, often shutting down altogether at 50% or 60%. I did not have AppleCare.
I called Apple, and they said they don't replace batteries (they are "consumables"). Later in the month I bought Apple Care for the MacBook Pro (within the one year requirement). Then, I called Apple back, and low-and -behold they were now willing to replace the battery. They overnighted me a new battery for free and I just had to send back the old one in the box.
So, if you have AppleCare, and the battery has low health (like 50% or lower) after a year (with 100 or less cycles), then you can get a replacement. Low cycles and the computer turning off while still saying it has 50-60% power are sure signs you've got a defective battery.
With no AppleCare, good luck getting them to replace it.