To realize how it is difficult to observe the USB spec, is the start point to design a charger using USB VBUS.

As for the USB side, (usb_20.pdf 7.2 Power Distribution p171)

USB VBUS Voltage/Current
a) voltage 4.75 - 5.25V (high power port)
b) one unit = 100mA
c) A maximum of 1.0 second is allowed for an averaging interval of current.
d) The amplitude of the current spike cannot exceed the device power allocation, 100 mA (or 500 mA).

Device state
e) Until the device is configured by the host, the current is limited up to 500uA.
f) bMaxPower field of Config descriptor determines the device current request. (up to 250 = 500mA)
g) When the device current request exceeds the host (hub) capacity just on the configuration, the device fails to be configured.
h) Devices begin the transition to the Suspend state after they see a constant Idle state on their upstream facing bus lines for more than 3.0 ms. The device must actually be suspended, drawing only suspend current from the bus after no more than 10 ms of bus inactivity on all its ports.
i) During suspend, if the device is configured for high-power and enabled as a remote wakeup source, it may draw up to 2.5 mA. otherwise, it is limited to 500uA.

Misc.
j) It is not allowed to equip more than one of upstream link ports or cables to single device.

USB 2.0 Specification
http://www.usb.org/developers/docs/