To slightly contradict Dave (slightly)
If you use a latop on AC power, it will constantly feed power through the batter (like riding the bike, it always charges the battery, if it needs to or not).
Yes, Li-Ion batteries have a better memory, but they still do degrade over time.
I haven't ever heard of anyone taking the battery out when using AC power, nor have I heard of anyone being told not to use the laptop whilst it's charging (again, imagine not riding your bike whilst the battery was charging! Stupid idea!)
IMO, use it, charge it at the same time etc. I would just try to always use the power from the battery unless it needs charging. That way, you're power cycling the battery better, and it'll keep it's memory a lot longer.