This is incorrect. There is no bidirectional negotiation between chargers and devices, nor are there any magic extra pins (at least for pretty much all Android and Apple products - dunno about Zune).
What there is is one USB charging standard, that basically says one thing and one thing only (that matters): if the data pins are shorted together (but otherwise not connected to anything), then the port is a Dedicated Charging Port. A DCP must meet certain voltage/current curve ranges and may be engineered to supply anywhere from 500mA to 1.5A (or more), with the voltage dropping as the device exceeds the charger's maximum. Devices are simply supposed to regulate current draw upwards until the voltage drops below a threshold, indicating the charger's capability. No digital negotiation takes place. Devices are limited to 1.5A charging current, which is quite typical for modern devices (and significantly better than the 500mA of a non-charging port).
There is a newer USB Power Delivery specification that is much more recent, supports higher powers, probably uses more complex negotiation (I haven't read it), and nothing implements it yet.
Then there's what Apple does - they have an incompatible implementation that uses resistors on the data pins in the charger to signal its current capability. Different resulting voltages mean different current levels. This is completely incompatible with the USB charging standard. Recent Apple devices (since the iPhone 3G or so) do support DCP chargers (to some extent - some charge more slowly, and I don't know about larger iPads?), but non-Apple devices will only charge at 500mA or worse from Apple chargers.