Hi Kev:
All of the USB cables that end with either the 'USB Mini' connector (the one that is in the battery bay of the Zumo) or the newer 'USB Micro' connector supply a 5 volt charge to whatever they are plugged into.
The amperage supplied will vary, depending on where the cable gets its power from. If the cable is plugged into a USB port on a laptop computer, the amperage may be as low as 500 mA (half an amp). If the device is a power transformer that plugs into the wall, for example, something you got to charge your phone, Blackberry, or camera, it might be 700 mA or even 1,000 mA (one amp).
I don't think that the amount of amperage supplied matters very much - obviously, it will affect the length of time it takes to charge the device, but beyond that, I think just about all devices (Zumos, cell phones, etc.) will accept whatever amperage you give them.
Michael