When you're editing a node which has the "Provide a block" option enabled the "Provide a block" filedset just shows the current theme's regions. I think it would be better if it showed all the regions of the actives themes . I tried to create a patch but I found very dificult to call all the theme_regions functions because you have to include the template.php files, and some template.php files would have functions with the same name, like phptemplate_body_class or phptemplate_breadcrumb. Best practices says that you should'nt use name's functions like theme_engine_function_name, but how do you check that?