battery depletion after 11 months

i bought my MBP last july 2007, and just recently i noticed the fast discharge of my battery when not plugged in. According to iBatt and coconut battery, my current battery capacity is 4954 mAh and the original battery capacity is 5600 mAh (88% capacity) is this normal? or do i need to have my battery replaced. I also calibrated my battery just 2 days ago. I always keep my MBP always plugged in and use the battery whenever im away. having the MBP less than a year the battery lost a significant amount of capacity. what should i do? thank you.