Even though it can't be overcharged, the lifetime of the battery is slightly shortened if it stays on 100% charge _at all times_. At least in combination with some heat.

That's why laptop batteries simply don't last at all if you keep them on the mains all the time -- they're a bit warm inside, as well as being at 100% charge.

Lifetime is drastically improved if it's a bit cooler though, so it's not as bad as that for a device like the N800.

More on this on the batteryuniversity link in the posting above.