To continue:
I believe that a battery is in a constant state of discharge no matter what. This occurs whether or not the battery is plugged in or by itself on a table. The addition of heat to a batter, however, increases that rate of discharge, thereby forcing you to charge it sooner than intended. With that in mind, perhaps it is killing the battery at a faster rate to be in the machine charged as opposed to by itself in a cool spot.
EDIT: Sorry, simple answer:
Yes, your battery is discharging constantly, to include while plugged in and "fully charged" in your macbook. Evidence suggests that your battery will wear down less quickly if away from the heat of your machine, but since it isn't a big story all over the computer world I doubt that it's that much of an issue. Your battery's life is primarily decided by how often you cycle it (use the charge, charge it back up).