[Zope] Automatic content expiration in the CMF...

I was wondering if anyone has tackled this. I thought that Zope would automatically expire content (set the review_state to private), once the expiration_date on the content is passed. How can I do this with DCWorkflow? I've tried to set up an automatic transition, but it doesn't do anything. Do I have to write a background task to do this, and if so, how is that done? Thanks, Jeff