You know, i alwasy believed it was fine to leave a battery in a laptop even if it was left on charge. I remember my dad asking me if it was o.k. and i said sure it is....
my dad used his laptop "normally" for about 3 months, allowing it to discharge and then recharge.
Then he left it on on the mains for almost a year. Now the battery has no life what so ever in it. I blamed it on a faulty battery, and went to the apple store waving the apple care. The genius at the store eventually ask if it had been on chrage for a long period. When i said 18 months, he laughed and siad thats why. He tried to explain about it needed to be discharged once in a while to ensure it remained correctly callabrated. And that it would slowly loose power over time and then it would charge a little and this would add up to wear and tear.
To be honest, i think its bull, but thats what the "genius" told me. I personally think its fine to leave it in for a while... maybe just not 18 months.