as much as there is a warning to plug into a charger, there is one to disconnect it once full.
i try not to do either.
i believe someone (might have been chobosandwich) explained it in an old thread. about "vampiric drain" where each charge is a cycle, the life of a battery will do so many cycles....leaving it connected means it charges for a little. stops, run down a little, then charges a little, repeat...hence, using up those cycles....just REALLY slowly.
wont damage it, just wont last as long as some others in the "technical" long run, which to me wont be a large difference.