Others have posted that the Pi with nothing else plugged in and just idling it draws ~330 ma (~300 ma was originally reported by Eben, IIRC), with a keyboard it draws ~385 ma, a keyboard, mouse, and network cable it draws ~450 ma (three LEDs lit), and when various ARM CPU-intensive software is run at up to 99% load in that latter fully-connected setup, it drew up to ~485 ma. It hadn't been tested with the GPU running full-bore, yet, but, the remaining ~215 ma to get to 700 ma maximum is otherwise available for other USB devices.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!