Is it possible that your nokia chargers have faulty contacts in them? Do they charge any other device? I've burnt one male microusb-connector when I had some crap in the connector. It wouldnt get good connection before I scraped the melted plastic of the pins/contacts. Also some melted plastic could be on the N900 connector and the usb-cable might be in better condition and get the connection, but more worn out charger connectors might fail. (Ok, this wouldn't explain the no power in usb-mode)