For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge. This keeps the battery juices flowing.
Apple does not recommend keeping the battery plugged in 100% of the time only because you will not be letting the juices run occasionally, not because of fear of overcharging.
They give the example of during the commute to work using your notebook, and charging it while at work and then when leaving work using it on battery power.
This is allowing the juices to flow as when the notebook is on charger fully charged the batteries juices aren't flowing, meaning that it is running of mains power, just as is the case with iPhones, iPods, etc.
If you are using your notebook on mains power all the time, and if not letting it run off battery at all, it is recommended to calibrate the battery once a month.
This gets the juices flowing.
If there was a fear of overcharging, Apple would say do not keep the notebook plugged in once the battery charge is at 100% because it will overcharge, or damage the battery.