I've got Powerjolt Dual Universal 2 USB Blk Car Charger, and it works sometime that's really weird but I don't know how do I get it working. Plugging and unpluging the cable few times. Most likely I have to plug the charger, than cable into charger and finally cable into n900. and yes, I do use out of the box cable.

btw, this charger doesn't work with 5800, so it could be charger issue too.