rsdio tipped us to an app note on USB-port power:
Very cool! This is like an audio ducking compressor for voiceovers, but for charging.
This circuit exploits all the power available from a USB port by dynamically adjusting the amount of current delivered to the load, thereby siphoning a relatively constant (and maximum) current from the USB port. Included are a current-sense amplifier (MAX4173), voltage reference (MAX6129), and precision op amp (MAX4238).