The other day I noticed my current battery capacity was around 65%, which for a MBP under a year old I was a little annoyed at. According to iStat Nano, a [excellent] system monitor widget and confirmed with Coconut Battery the maximum battery charge I could currently hold would be around 3500mAh instead of the factory default of 5500mAh.
After a quick search on the Apple site I have managed to bump it back up into the 90% region. How? Easy, I Calibrated the battery, something you should do every 2 months or so!.