I can'y be 100% sure about this but I think the N900 relies on a driver to tell it to start charging. If I plug the usb cable into a machine that has no driver - it won't charge - as soon as the driver is present, away it goes!

I assume Nokia have used some extra wizardry similar to various apple goods, whereby an extra contact in the usb supplies a different signal to kick off charging. Though I would have expected something that worked with an apple device to do the same with the N900!

It would be interesting to see if anyone knows the real answer to this! I'm not sure how their charger adapter in the box can work with other chargers - unless it has the extra signal wired in. Have you tried attaching that to any of your chargers or do all your chargers have the correct plug already?

I'd definitely be interested to know about this, as it could mean all sorts of chargers and extra power packs might become troublesome!