How many cycles are on your battery?
When was the last time you calibrated?
Go to

>>About this Mac>>More Info>>Hardware>>Power
To the right you will see your battery's (the one that is currently in the computer) information.
As you can see from the picture, my battery currently has 29 cycles on it.
Check the total number of cycles and your maximum potential charge versus your current charge (if the battery is displaying 100% charged) (for my battery, you'll see that the figures are 5282 for the maximum attainable charge, and 5243 for the current charge held, which equals 99.3%, or perfectly healthy). If these numbers vary drastically, your battery is likely to be wearing out. However, this is expected as time goes on and as the number of cycles increases. The battery should retain 80% of its maximum charge all the way up to 300 cycles.
Read more about your battery and maintaining it
here.
As for Applecare, since you have it, Apple will replace a faulty or defective battery. Most of the time, they will also replace a worn down battery (ie, you've used it for more than 300 cycles), but they aren't obligated to do this under the terms of Applecare. However, most geniuses and store personnel are willing to be accommodating.