The previously proposed convention of storing dispatch args in the 'wsgi.url_vars' environ key has now been marked as accepted but the key has been renamed to 'wsgiorg.routing_args'. That means that this is no longer an experimental feature and you can rely on it going forward. That was quick!
environ['wsgiorg.routing_args'] == (list_or_tuple, a_dict)
See the Selector page for more info.