Chris Hartjes has had a few posts about "metatesting" already and in this latest article he takes the series one more step. He looks at moving outside of the current toolset and expanding on them to meet your testing needs.
While PHPUnit is awesome out of the box, it still lacks some tools that are required to do things like test protected class methods or assign values to protected class attributes. Lucky for me we have an awesome testing engineer at Kaplan named Will Parker who has shown me some ways that they have extended PHPUnit itself to make testing certain things easier.
Chris talks about things like testing protected methods (easy thanks to a helper) and checking the value of a class property. The key to both of them lies in using PHP's own Reflection functionality to alter properties on the class objects themselves.