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).
just tap the 5V line from the PSU.
yeah but this gives you a power and data line all in one, and with the battery as a acumulater you could have much higher peak currents(couple of AMPS, as long as average consumption over a lenghty period is below is below 500mAmps.
and the peak currents can last for a long periods maybe hours at a time.