I just got an n900 today, I charged it to green (100%), flashed it, then used it for several hours until the battery was nearly dead. I put it on its charger, left it for a few minutes, then it said that it needed to be recharged! It shut down, and now all it does is boot loop to a dim "Nokia" screen while on the charger, and while off of it won't turn on at all.

I have a similar problem. Mine is one of the Hong Kong fake new phones which was actually a refurb of a broken phone that bricked after taking 43% of a flash. Can you reflash or try an external charger?

I don't have an external charger yet. I flashed the phone successfully, luckily that worked.