It doesn't - therefore, a to-spec USB2.0 host should not give more than 100mA over the condom, but good luck finding such a beast (most hosts will happily serve 500 mA w/o negotiation).
Nitpick: It's current negotiation, not voltage negotiation. As for voltage - 5V is universal with USB.
And as for current - I wrote about the matter in another reply to the OP: The problem will be a well-behaved USB device simply won't take more than 100mA. I.e. it will use the "flat" charging curve, even if the host could deliver more.
Nitpick for interest's sake: apparently there's a new (2012) 'USB Power Delivery' spec[1] which specifies two new voltage levels (12V & 20V) in addition to 5V, with higher current limits (2A at 5V, 3A at 12V or 20V for microUSB connectors). Obviously both ends (and the cable) have to support it; I don't know if anything actually does yet.
Pedantry: That should be obvious from the units, wouldn't it? (Volts for voltage, amperes for current, ohms for resistance) I do agree that the general public uses these interchangeably, bringing confusion :)
As for a well-behaved device - there aren't very many of those, either. Fortunately, this is less of a problem with USB3 - more devices follow the spec closely.
