Martin pretty much hit on the technical details. I have been trying to standardize a subset of my gadgets on USB for years simply because having the connectors all the same gives some versatility in charging them.

But the fact remains that just because it has a USB plug does not mean it uses all the USB standards; and while most such devices will charge when plugged into a computer's USB port, it can take much longer to charge than if you had used a charger connected to a 110v or 12v power source.

After Irene I used a battery booster pack with a 12v outlet to charge our cell phones. The car charger used for this has a wire with a micro-USB connector, as well as a standard USB port. By using another USB cable with a micro-USB connector I could charge both phones at the same time. However, the wire coming directly from the charger did the deed twice as fast as the cable from the USB port; presumably because the port was throttled to the voltage/amperage of the USB standard.