I had a simliar experience.

First day with the N900 it charged with the wall charger. Then back and forth the wall charger would work one charging session, then not another (the USB cable to connect to a laptop or desktop consistently worked, just not the wall charger). I used the same outlet each time so it's not that. The sessions where the wall charger failed, the N900's screen DID light up upon plugging the wall charger, but the battery wouldn't charge (no usual "charging" banner across the screen, and the battery indicator bars didn't rise as it normally does as it's charging).

I was close to sending a refund request to exchange the product but decided to look around in this forum first. I didn't find any similar experiences here but since then my wall charger has worked fine (knock on wood). That's less than two weeks of owning this device.

Maybe you should exchange it for a working product.