the droid appears to follow the USB spec (unlike MANY other devices) and it first connects to a usb port at only ~100 mA (which might barely charge faster than it uses power). Then it ASKS the host device for permission to use more power. I think it steps to 300 or 500 mA IF the host device says it's OK. If the host device doesn't reply that it's OK then the droid will not draw more power.
if the laptop feels it doesn't have power to spare it will tell the droid to stay at 100 or only step to 300.
for comparison the stand alone wall wart supposedly outputs 850 mA.
when hooked up to a 3000 mA charger so it can suck it's max, the droid seems to pull something like ~1500 mA