Comment 1 for bug 694986

Is there a reason you are not using plone.testing (http://pypi.python.org/pypi/plone.testing) to get proper ZODB isolation?

plone.testing and it's Plone integration layer plone.app.testing have been written to fix many of the shortcomings in zope.testing's layer approach and provide useful layers and helpers for testing Zope and Plone. Going forward I expect ZopeTestCase and the products based on it like PloneTestCase to go away and be replaced with the newer approach.