Guys

I don't have all the details at my fingertips right now but it is not as simple as you think. When a USB device connects to a computer, it negotiates with the host to set the proper current.

There is a USB standard specification for how to implement a "dumb" charger, I think that it requires the D+ / D- lines to be shorted together or connected by a certain resistance. The N900 expects either to talk to a USB host to set current or it expects to connect to a properly configured "dumb" charger. The problem occurs when the "dumb" charger does not implement its USB interface according to the spec. Most likely the Apple charger does not implement the spec correctly. But I too have a mishmash of USB charging devices and some work, some do not with the N900.