The version I posted 2 days ago might not have worked with the normal Uno USB driver. If anybody actually tried it and ran into this problem, I apologize! I have corrected the problem, and added the source code. The link is the same as before.
http://timgiles.free.fr/arduino-usbisp.zip
The compilation procedure is the same as for the standard Arduino USB code, and requires LUFA v100807 and WinAVR v20100100.