A USB power bus (by definition) is rated for 500mA at 5V for the whole bus, meaning that if you exceed that draw then the power bus will shut down. Some systems may have more available power, but that is the minimum requirements for USB. 500mA is the total for every device on the USB bus not for each device so pay attention to that.
A typical single 80mm fan at 12V draws around 250mA and even more during startup, so I bet you will be on the edge of overdrawing your USB source with a 5v fan which can range from 200mA to 600mA in the sizes we are talking about for the 360. Just pay attention to the numbers, i believe you wont damage anything if you exceed the numbers as USB will shutdown if overcurrented.
