fro the arduino website
USB Overcurrent Protection
The Arduino Mega has a resettable polyfuse that protects your computer's USB ports from shorts and overcurrent. Although most computers provide their own internal protection, the fuse provides an extra layer of protection. If more than 500 mA is applied to the USB port, the fuse will automatically break the connection until the short or overload is removed.
source: http://arduino.cc/en/Main/ArduinoBoardMega
so (if i've got this right) the usb won't ever deliver more than 500mA.