The new dialog interface uses a new format for Dialogs. By default you can view this at: /libs/wcm/foundation/components/page/cq:dialog
This can be overlayed in the same way as the other dialogs.
You can also preview the dialog because it is a component of type (cq/gui/components/authoring/dialog): http://localhost:4502/libs/wcm/foundation/components/page/_cq_dialog.html/content/geometrixx/en/jcr:...
The part after the _cq_dialog.html/ is a suffix of the page to pull in the properties for.
There is some docs on the Coral UI that is the front-end of this: http://dev.day.com/docs/en/cq/current/touch-ui/coral-ui/
Not sure how to get around the double dialogs for pages. For components it appears to revert to the Ext JS dialogs if a cq:dialog component isn't present.