Raphaël:

This branch looks very good. I think the implementation is perfect. I have some concerns about the primary test.

> @@ -260,3 +267,79 @@
>                  text='Series and milestones'))
>          self.assertThat(view.render(), series_header_match)
>          self.assertThat(view.render(), Not(add_series_match))
> +
> +
> +class DistroRegistrantTestCase(TestCaseWithFactory):
> +    """A TestCase for registrants and owners of a distribution.
> +
> +    The registrant is the creator of the distribution (read-only field).
> +    The owner is really the maintainer.
> +    """

This test case will be difficult to extend. I think we want tests for
the model object and tests for the views. I preffer that each view (+add
and +reassign) be have separate test cases, but that may not be necessary.
Some of these tests need to move to a different directory.

    Test interfaces in lp/registry/doc
    Test models in lp/registry/tests
    Test views in lp/registry/browser/tests

Get the rest of this test case updated per my remarks that split it into
model and view tests.

> +    layer = LaunchpadFunctionalLayer

This is the wrong layer. LaunchpadFunctionalLayer is slow and only needed
if you are working with the librarian (uploading/downloading/email messages).
Use
    layer = DatabaseFunctionalLayer

+    def setUp(self):
+        super(DistroRegistrantTestCase, self).setUp()
+        self.owner = self.factory.makePerson()
+        self.registrant = self.factory.makePerson()
+        self.admin = getUtility(IPersonSet).getByEmail('<email address hidden>')
+        self.registry = getUtility(IPersonSet).getByName('registry')

Let's not ever use sampledata. You do not need the admin or registy attrs.
You can use login_celebrity('admins') or `with celebrity_logged_in('admins'):`.
Both login functions create a user that is a member of the celebrity team,
'admins' or  'registry_experts', and *only* that team. The sample data
users are members of many teams and the permission may not be as you assume.

+    def test_registrant_owner_differ(self):
+        distribution = self.factory.makeDistribution(
+            name="boobuntu", owner=self.owner, registrant=self.registrant)
+        self.assertNotEqual(distribution.owner, distribution.registrant)
+        self.assertEqual(distribution.owner, self.owner)
+        self.assertEqual(distribution.registrant, self.registrant)

^ Model test. This is fine.

+    def test_registrant_set_by_creation(self):
+        """The registrant field should be set to the Person creating
+        the distribution.
+        """
+        login_person(self.admin)
+        distributionset = getUtility(IDistributionSet)
+        creation_form = {
+            'field.name': 'newbuntu',
+            'field.displayname': 'newbuntu',
+            'field.title': 'newbuntu',
+            'field.summary': 'newbuntu',
+            'field.description': 'newbuntu',
+            'field.domainname': 'newbuntu',
+            'field.members': 'registry',
+            'field.actions.save': 'Save',
+            }
+        view = create_initialized_view(
+            distributionset, '+add', principal=self.admin,
+            method='POST', form=creation_form)
+        distribution = distributionset.getByName('newbuntu')
+        self.assertEqual(distribution.owner, self.admin)
+        self.assertEqual(distribution.registrant, self.admin)

This test belongs in lp/registry/browser/tests. It overlaps with the
distribution-views.txt test. You can either update that test, or delete
the duplicate part and keep this. I favour keeping your test and
extending the assertions to include the assertions that the other test
did.

+    def test_reassign_change_owner_not_registrant(self):
+        """Reassigning a distribution should not change the registrant."""
+        login_person(self.admin)
+        distribution = self.factory.makeDistribution(
+            name="boobuntu", owner=self.owner, registrant=self.registrant)
+        reassign_form = {
+            'field.owner': 'registry',
+            'field.existing': 'existing',
+            'field.actions.change': 'Change',
+            }
+        view = create_initialized_view(
+            distribution, '+reassign', principal=self.admin,
+            method='POST', form=reassign_form)
+        self.assertEqual(distribution.owner, self.registry)
+        self.assertEqual(distribution.registrant, self.registrant)

^ view test.

+    def test_reassign_page_title(self):
+        """Reassign should say maintainer instead of owner."""
+        login_person(self.admin)
+        distribution = self.factory.makeDistribution(
+            name="boobuntu", owner=self.owner, registrant=self.registrant)
+        view = create_initialized_view(
+            distribution, '+reassign', principal=self.admin, method='GET')
+        header_match = soupmatchers.HTMLContains(
+            soupmatchers.Tag(
+                'Header should say maintainer (not owner)', 'h1',
+                text='Change the maintainer of Boobuntu'))
+        self.assertThat(view.render(), header_match)

^ view test.