This can come in handy both from the Project onLoad script (if you want to start your MockServices when the project is opened) or from a TestCase/TestSuite setup script (if you need the MockService running for your functional tests). Here from a TestCase setup script:
def runner = testCase.testSuite.project.mockServices["My MockService"].start()
context.mockRunner = runner
The returned runner object is required to stop the MockService... so save it to the context and use it in the tearDown script:
Have a look at the WsdlMockService and the WsdlMockRunner classes for more methods and properties that might come in handy.