Theres at least 2 sides to this question, and its been asked and debated for as long as theres been laptops. As far as the pure hardware goes, once the battery is charged the current just passes through it/by-passes it for want of a better analogy. ( I'm not a tech writer)
Except for the time spent actually driving in my job (see my username), my Lappy stays plugged in 100% of the time. Have done this with my last 2 and current laptop (see spec.'s), with no noticeable degradation in my battery. I repeat, NO noticeable ill effects. From any of them , Compaq, Toshiba, Asus.
If you're worried about it, uplug it. Or take the battery out while connected. But IMHO, you don't have to and to me its just an unnecessary hassle.