OK, here it is: http://timgiles.free.fr/arduino-usbisp.zip
The zip-file contains the hex file, instructions, some useful pictures, and some bat files which should make the whole procedure relatively painless.
Note that this is a first version, and may contain bugs. Read the warnings and caveats in readme.txt.
Please report your successes or failures here. Once a few people have got it working I'll post a proper write-up on the Playground.