I used metaplugin, sorry.

But it is reproduced with openvswitch plugin. (I used current stable/folsom.)

operation log:
---
$ quantum router-create routerA
Created a new router:
+-----------------------+--------------------------------------+
| Field                 | Value                                |
+-----------------------+--------------------------------------+
| admin_state_up        | True                                 |
| external_gateway_info |                                      |
| id                    | 6cf18db6-3b0c-478f-9aec-f395a4f819e6 |
| name                  | routerA                              |
| status                | ACTIVE                               |
| tenant_id             | 941416a1628640228162415d8f13b2f1     |
+-----------------------+--------------------------------------+
$ quantum router-gateway-set routerA net-ext
Set gateway for router routerA
$ quantum router-interface-add routerA subnetA1
Added interface to router routerA
$ quantum floatingip-create net-ext
Created a new floatingip:
+---------------------+--------------------------------------+
| Field               | Value                                |
+---------------------+--------------------------------------+
| fixed_ip_address    |                                      |
| floating_ip_address | 192.168.0.201                        |
| floating_network_id | c7a5b64b-6a7e-4837-8022-457a316832f3 |
| id                  | e79477d1-0815-49b0-b32f-cbdc7cd3761c |
| port_id             |                                      |
| router_id           |                                      |
| tenant_id           | 941416a1628640228162415d8f13b2f1     |
+---------------------+--------------------------------------+
$ quantum port-create netA1
Created a new port:
+----------------+---------------------------------------------------------------------------------+
| Field          | Value                                                                           |
+----------------+---------------------------------------------------------------------------------+
| admin_state_up | True                                                                            |
| device_id      |                                                                                 |
| device_owner   |                                                                                 |
| fixed_ips      | {"subnet_id": "0c2fd71e-1c52-4876-b296-b0a315f08f60", "ip_address": "10.0.1.3"} |
| id             | 1196d979-536f-4ca2-8b50-2abf5c075e1a                                            |
| mac_address    | fa:16:3e:90:57:42                                                               |
| name           |                                                                                 |
| network_id     | d44bba19-babe-49cd-9897-22ac107efcf1                                            |
| status         | ACTIVE                                                                          |
| tenant_id      | 941416a1628640228162415d8f13b2f1                                                |
+----------------+---------------------------------------------------------------------------------+
$ quantum floatingip-associate e79477d1-0815-49b0-b32f-cbdc7cd3761c 1196d979-536f-4ca2-8b50-2abf5c075e1a
Associated floatingip e79477d1-0815-49b0-b32f-cbdc7cd3761c
$ quantum router-interface-delete routerA subnetA1
Removed interface from router routerA
$ quantum router-delete routerA 
Request Failed: internal server error while processing your request.
---

quantum-server log:
---
2012-11-20 08:00:19    ERROR [quantum.api.v2.resource] delete failed
Traceback (most recent call last):
  File "/opt/stack/quantum/quantum/api/v2/resource.py", line 96, in resource
    result = method(request=request, **args)
  File "/opt/stack/quantum/quantum/api/v2/base.py", line 360, in delete
    obj_deleter(request.context, id)
  File "/opt/stack/quantum/quantum/db/l3_db.py", line 243, in delete_router
    context.session.delete(router)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 449, in __exit__
    self.commit()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 361, in commit
    self._prepare_impl()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 340, in _prepare_impl
    self.session.flush()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 1718, in flush
    self._flush(objects)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 1789, in _flush
    flush_context.execute()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/unitofwork.py", line 331, in execute
    rec.execute(self)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/unitofwork.py", line 498, in execute
    uow
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/persistence.py", line 115, in delete_obj
    cached_connections, mapper, table, delete)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/persistence.py", line 671, in _emit_delete_statements
    connection.execute(statement, del_objects)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1449, in execute
    params)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1584, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1698, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1691, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/default.py", line 331, in do_execute
    cursor.execute(statement, parameters)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
IntegrityError: (IntegrityError) (1451, 'Cannot delete or update a parent row: a foreign key constraint fails (`ovs_quantum`.`floatingips`, CONSTRAINT `floatingips_ibfk_3` FOREIGN KEY (`router_id`) REFERENCES `routers` (`id`))') 'DELETE FROM routers WHERE routers.id = %s' ('6cf18db6-3b0c-478f-9aec-f395a4f819e6',)
2012-11-20 08:00:19    DEBUG [eventlet.wsgi.server] 172.17.190.3 - - [20/Nov/2012 08:00:19] "DELETE /v2.0/routers/6cf18db6-3b0c-478f-9aec-f395a4f819e6.json HTTP/1.1" 500 215 0.174298
---

Thanks.