Of course it is covered. I don't know about Nokia in particular, but power supplies are a relatively common source of failure.
The n900 charger is a standard micro-USB charger, so its easy to get a cheap replacement anyway. Cheaper than postage for a warrenty claim for those of us who had to order the phone from overseas.
I've had similar happen a couple of times, and it just took a phone call to get a replacement sent in both cases. But (in my experience also) "Nokia Cares" is an oxy*****. They probably want the faulty item mailed back with a statutory declaration and return postage paid :-(