I keep my on the charger when not in use too and have since the day it came, about a week after they were available.
So far, the battery gauge seems to indicate full charge time is about the same as it was on day one.
It has ran down a couple of times when I didn't realize it was sleeping instead of off and forgot to plug it in.
Once it popped up a shutdown warning and then shut itself off while I was using it, and the other time when I went to turn it on it woke up with a low battery warning so I turned it off and put it on the charger.
No apparent harm as far as I can see.