Lots of posts that won't do you much good -what a pain they are
V-usb creates a device that emulates a usb device of your choice where you can set the ID numbers , power requirements etc. One kind soul has altered this to run on Arduino with examples and it can be had here .
http://code.google.com/p/vusb-for-arduino/downloads/list
and here the thread about it
http://arduino.cc/forum/index.php/topic,123306.0.html
Other useful but voluminous materiat
http://www.beyondlogic.org/usbnutshell/usb1.shtml