Why won't my Android phone charge from my computers USB port and why does it seem it actually drains my battery?
To expand on Matthew's comments, from wikipedia (http://en.wikipedia.org/wiki/USB):
As with previous USB versions, USB 3.0 ports come in low-power and high-power variants, providing 150 mA and 900 mA respectively, while simultaneously transmitting data at SuperSpeed rates. Additionally, there is a Battery Charging Specification (Version 1.2 xe2x80x93 December 2010), which increases the power handling capability to 1.5 A but does not allow concurrent data transmission. The Battery Charging Specification requires that the physical ports themselves be capable of handling 5 A of current but limits the maximum current drawn to 1.5 A.
In other words, it's worth it to try as many sources as you can find when looking for a good charging port.