Bzzt.

You should try and keep it as charged as possible. The lithium-ion batteries in the tablets don't take well to frequent discharging (since they have a limited number of charged-discharged-charged cycles).

Basically, you want to try and balance between two extremes: Discharging it as little as possible, and keeping them cool. Cool is more of a problem with laptops as they generate a lot of heat. Since the tablets don't generate much heat, you'll tend to shift more towards keeping it charged.

I've owned both a 770 and an N800 since their respective release dates, and keep them both plugged in whenever I'm at home and at my desk. I haven't noticed any significant drop in battery life on either device, so charging overnight is obviously not the harmful practice that Nokia's documentation and many people here make it out to be (this is something that dates back to the NiMH days and does not apply to Li-ion).