Actually, its not completely straightforward. USB specs only require 500mA. As such, devices that can consume more current need to know the USB host it is plugged into can provide more than 500mA so that it can safely request it. AFAIK, there are 2 methods for this and from my own testing on the Incredible, DX and Bionic, there is no consistency between manufacturers. The Incredible used a simple data signal short, while Motorola used the more complicated version outlined in the USB spec. Where this becomes relevant is that the Incredible could not draw more than 500mA from a dedicated charger that used the more complicated signal while the 2 Motorola phones drew 750mA. Conversely, the HTC pulled 850mA from its OE charger while the Motorola phones would only pull ~650mA from the Incredible's charger. Moreover, even chargers rated to 1A or even 2.1A would not yield their maximum currents. It would appear the phone will limit the current to a specified maximum regardless of what it is plugged into. Admittedly, my testing was brief and not comprehensive enough to test all states of the phones, but it would appear that there are differences that may have a drastic effect on the charging times despite the current ratings on the charger.