Some days ago we had the requirement to display some entityforms and other Drupal Core forms like the login in modal windows. The second requirement is easy to solve. Many modules like colorbox.module, lightbox.module and others solve this already.
Sometimes you may want to check if a user has permission to access an EntityForm. Perhaps to find out, if you can display a block, link or special advice. This is not quite as sinple, because always returns true for the EntityForm URLs, which is not good but... OK, we have to deal with it. So what's the right way? Use "entityform_access()"!