In the recent bakery article concerning the ongoing development of CakePHP 2.0. The already underway migration from SimpleTest to PHPUnit was introduced. I wanted to go into some of the reasons and motivations for that decision as well as explain some of the long term benefits.
He lists some of their thoughts behind making the move including the fact that SimpleTest isn't E_STRICT compliant and that the integration of PHPUnit into some of the popular continuous integration and deployment tools makes it an obvious front-runner. Of course, there'll be a bit of pain as the transition is made, but the new tool will give much more insight into the CakePHP codebase. Most of the tests will translate relatively easily, though.