I think you are missing the point. Yes, once the battery is full, it switches back to battery. Fine. Yes, leaving the screen on uses power. Fine.

The key is that I have NOT uplugged the charger. I do not care if it is using power, because presumably, once the battery hits 85% (or whatever), it will start charging again, because it is still plugged in. It will then cycle between these two states until I unplug the charger.

While the N900 does exactly what it claims (screen on while charging) this is counter-intuitive behavior. I think most people expect that if it is plugged in, it is "charging", whether it really is or not. I believe my N800 screen will stay lit while on the charger (whether it is "charging" or not).

Come to think of it, that would make a great time-lapse video on a road trip using navigation software, watching the screen go blank, the green led light up, then go out, the screen light up, and repeat. I'm surprised Nokia didn't do it for one of their usability videos. A bunch of Finns in a car cursing because they missed a turn due to lack of voice guidance and a screen that turns off once the N900 is fully charged, so they can't see or hear the turn they missed.