I've noticed this error message a couple of times as well. But in my case, I keep the N900 topped off when I'm out and about via a 12V @1A DC charger (a car cigarette lighter charger). The N900 specs call for a 1.2A (1200ma @ 5V current). It charges fine so long as the battery is not *too* low. When the battery is low, the charger doesn't supply enough juice to satisfy the N900's charging driver, and it pops up the "Not Charging" message after a few seconds of it being plugged in.



Why it's doing it when using it with the supplied AC charger as in the OP's post...? ~shrugs~
In my case, the AC charger seems to be able to keep up with the phone (yes, yes, "It's not a phone!"), and charges it just fine.