The tests are defined in Excel. I then parse the excel file (using jython and POI) to produce jython test case methods (making use of the python unit-testing framework). I can then manually tweak the test cases if required, but the vast majority are ready to execute directly from the Excel data.

Using excel rather than some XML format or other interface is much better. Excel is defintely the fastest data entry interface that I have used and everyone feels comfortable with it. By getting business users to define the business rules in an agreed format, I have a very good test suite built for very little effort.