I have two devices a Nokia E7 and an HP Touchpad both of which are charged using the USB port.
They will both charge using the provided AC adapter and from a computer USB port using either the provided USB cable or any other USB cable. However they will not charge from a generic USB power supply, I have two an AC one and a car one. It would be very handy to have just one unit that works in the car ad be able to use what I already have.
It appears that the Nokia will charge from the Touchpad charger but the Touchpad will not charge from the Nokia charger.
How do you get a generic supply to work? I'm guessing that there is some sort of R/C network hung across the bus connectors on the proprietary chargers which the device recognises as the charger or a computer.