To cut a long story short, it's really hard to find a specification of the JUnit output file format, but after lots of trying, I had a script that could run simpletests and report the results in a JUnit friendly (Hudson friendly) format, yey!
Find the script attached, there's an extra argument from the normal set, --xml which allows you to specify the directory in which to output the xml result files. Lovely.
Finally you'll want to make a bash script that passes in all the right arguments to the script and then run that from Hudson, as part of the normal build process.