A USB power sink is supposed to (i) identify itself as a USB device, and (ii) announce that it wants to use more than 100mA, if it needs more than this base load which is available to every port. Since the Pi's micro-USB has no data lines connected, it can neither self-identify nor announce its power requirements, so it would appear to be non USB compliant.
This could be a serious problem if we want to power a Pi from a full-spec powered USB hub via a type-A-to-micro-B lead. The Pi should announce that it wants more than 100mA, but it can't. ....
It occurs to me that there might be an exemption from USB spec compliance specifically for dumb chargers that are not really USB devices at all, but just use the micro-USB type B plug. Does anyone know if this is the case?
It worries me that there might indeed be such an exemption, the end result of which could explain the dire state of cellphone chargers that we have witnessed. They shouldn't really be regarded as USB-spec power supplies at all.
In any case, the fact that the Pi is not USB compliant for powering from a fully compliant powered USB hub is an issue to consider.