Whoops, here's an unfortunate UX hiccup in the implementation:

Here is how to edit someone else's mockup for your own post:

     1) edit the original question/answer containing the mockup you want to improve

     2) copy the part between
         <!-- Begin mockup
        and
        End mockup -->

     3) hit cancel

     4) .. more steps

This seems poorly planned to me. Instead, like the edit link, there should simply be an iterate link.

But otherwise, I think this is a very positive and helpful integration. Perhaps similar positive integration might be sites like JS fiddle that allow you to run code in the browser would be great integration for the programming stack overflows.