In a new post to the Debuggable blog, Tim Koschutzki talks some about the work he's been doing on the code coverage analysis for the upcoming CakePHP release:
There are several different kinds of criteria to code coverage. The two most important ones are line coverage (or statement coverage as wikipedia puts it) and path coverage. [...] So how is it going to work? Pretty simple actually. Whenever you run a CakePHP test case Cake assembles information in the background about which lines of your subject-under-test are called.
He includes a few screenshots of the code coverage being run and the end result of the runner with the percent covered.