Everything needed is included here
http://code.google.com/p/unojoy/
and here
Yes the Arduino's USB interface has to be flashed. This is done with Atmel's FLIP tool.
There is a Arduino-keyboard-0.3.hex available here:
http://hunt.net.nz/users/darran/weblog/b3029/Arduino_UNO_Keyboard_HID_version_03.html