From the User Guide...
"Do not leave a fully charged battery connected...overcharging may shorten its lifetime."

Well, so may unnecessary* discharge cycles. So should we plug it in for use?

* 90% of use will be next to an AC outlet

I always leave mine plugged in - no problems yet, and I never had any problems with my 770 which I would leave plugged in when fully charged before being replaced after about 16 months of sterling service by a N800.

Overcharging is not an issue with lithium batteries (more below). The warning in the Nokia docu is some rubbish left over from way back when they used NiMH and NiCd batteries.

Lithium batteries cannot be allowed to overcharge: They explode! They burn. Very very dangerous. That's why devices using lithium batteries have internal charging circuitry which will switch off the charging when the battery voltage reaches a certain limit. My Palm T3 switches off at 4.24 volts, some other devices will switch off at 4.10 volts. A lithium battery must never go above 4.30 volts. This is so critical that there is an _additional_ over-voltage circuit inside the battery itself, in case the built-in one in the device fails (except for the batteries you can buy in bulk for RC purposes, for hobbyists).

Unlike with NiMH batteries there is no trickle-charging. Instead the charger won't start up again until the voltage of the battery has dropped a good bit.

Don't discharge the battery. The discharge cycling is again a leftover from NiMH (and in particular NiCd) batteries. Charge it when you get the chance. Deep-discharge a lithium battery and you will see reduced capacity right away (been there, done that).

Lastly, it _is_ some truth in that a lithium battery that stays at 100% charged at all times will get a somewhat reduced lifetime. But this is much more of a problem if the battery is warm too, so in laptops this is a much bigger problem: Keep your laptop on the mains at all times and after a year your battery is pretty much spent. Much less of a problem with phones and devices like the N800 (as long as it doesn't get warm).