Nokia uses the MicroUSB charging standard, which requires the charger to identify itself as a charger by shrorting the datapins together. The included cable is a datacable, which does not modify the signals at all. If the charger itself does not comply to the usb chargin standard, the n900 wont charge.

There was another thread here about external battery packs, where people had modified usb extension coords to make n900 think the other device is a charger. Might be risky if used improperly.