Most computer USB ports supply 500 milliamps of current. Some computers have more than one USB controller to support the number of USB connections, potentially giving you two 500 milliamp supplies from one computer.
There is quite an in depth article about USB power and how devices (including iPods) can make use of it here
It suggests that iPods use almost all and sometimes more than the 500 milliamps supplied by the USB specification when charging.
Nikkor AF-S DX; 18-105 f/3.5-5.6G VR, 55-300 f/4.5-5.6G VR, 35mm f/1.8G
Speedlight SB-700 http://keystrokesukimages.com