I just looked at the data for the motors; they draw between 120mA at no load and 800mA (stall current). Hence in my application I'm probably only drawing about 150 to 200mA. So I could probably just manage using a regular 9v battery for the motors which can supply about 200mA to 300mA... seems like running more than 1 or 2 servo motors is going a bit of a hassle... :-s
Quick question though; is there any reason why it works fine when being powered by usb?