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.
I'm glad to have confirmation that the battery does not allow constant charging if plugged in. I leave the battery in my server iBook as a form of poor man's UPS for power flickers, but have always wondered how long the battery would hold out like that. Killed an HP battery years ago by abusing it with constantly being plugged in.