From Mike's update:
The most recent version committed to the mutation_testing branch is in a testable state. I would encourage anyone interested to try out the new mutation testing feature in this branch.
You can get complete information about the project - its methods and goals - from this page on the PHPUnit wiki. If you'd like to get this new code a try and see what his mutation testing is all about, here's a link to the subversion repository to grab the latest copy.