How about using the nokia charger (supplies 5.7 V) as the power source. It might be a tempting alternative. Anyway, whenevery you are near a usb keyboard, power can't be that far away. Although for using it as a card reader, 9v battery is much saner choice.

Also, a splitter could be made so that you can connect the charger to 770 at the same time as it's powering the usb.