Glad that Kees pointed out how easy it is to hack on USB stuff these days. The technology is available to anyone who has $25 (or less) and wants a programmable USB device for debugging or testing.
The "Teensy" AVR USB-stick is one such device. Write some code in the Arduino IDE and you can emulate mice, keyboards and such. If you want to get faster or more sophisticated, use C and LUFA, which can emulate USB storage devices and such.
(If you're in Portland, OR, you can even buy one at the biweekly DorkBot meeting.)