Two minor whitespace tweaks please, or rather the same one in two
places.  Other than that, awesome!

> === modified file 'lib/lp/code/interfaces/branchtarget.py'
> --- lib/lp/code/interfaces/branchtarget.py	2010-01-05 08:18:23 +0000
> +++ lib/lp/code/interfaces/branchtarget.py	2010-03-26 01:45:34 +0000
> @@ -88,6 +88,9 @@
>      supports_merge_proposals = Attribute(
>          "Does this target support merge proposals at all?")
>
> +    supports_code_imports = Attribute(
> +        "Does this target support code imports at all?")
> +
>      def areBranchesMergeable(other_target):
>          """Are branches from other_target mergeable into this target."""
>
> @@ -113,3 +116,18 @@
>
>      def getBugTask(bug):
>          """Get the BugTask for a given bug related to the branch target."""
> +
> +    def newCodeImport(registrant, branch_name, rcs_type,
> +            url=None, cvs_root=None, cvs_module=None):

This should be formatted like so:

    def newCodeImport(registrant, branch_name, rcs_type, url=None,
                      cvs_root=None, cvs_module=None):

(https://dev.launchpad.net/PythonStyleGuide#Wrapping%20long%20arguments%20in%20function%20definitions)

> +        """Create a new code import for this target.
> +
> +        :param registrant: the `IPerson` who should be recorded as creating
> +            the import and will own the resulting branch.
> +        :param branch_name: the name the resulting branch should have.
> +        :param rcs_type: the type of the foreign VCS.
> +        :param url: the url to import from if the import isn't CVS.
> +        :param cvs_root: if the import is from CVS the CVSROOT to import from.
> +        :param cvs_module: if the import is from CVS the module to import.
> +        :returns: an `ICodeImport`.
> +        :raises AssertionError: if supports_code_imports is False.
> +        """

> === modified file 'lib/lp/code/model/branchtarget.py'
> --- lib/lp/code/model/branchtarget.py	2010-01-05 08:18:23 +0000
> +++ lib/lp/code/model/branchtarget.py	2010-03-26 01:45:34 +0000
> @@ -19,6 +19,7 @@
>  from lp.code.interfaces.branchcollection import IAllBranches
>  from lp.code.interfaces.branchtarget import (
>      check_default_stacked_on, IBranchTarget)
> +from lp.code.interfaces.codeimport import ICodeImportSet
>  from lp.registry.interfaces.pocket import PackagePublishingPocket
>  from canonical.launchpad.webapp.interfaces import ICanonicalUrlData
>
> @@ -36,6 +37,12 @@
>      def __ne__(self, other):
>          return self.context != other.context
>
> +    def newCodeImport(self, registrant, branch_name, rcs_type, url=None,
> +            cvs_root=None, cvs_module=None):

Similarly.

> +        return getUtility(ICodeImportSet).new(
> +            registrant, self, branch_name, rcs_type, url=url,
> +            cvs_root=cvs_root, cvs_module=cvs_module)
> +
>
>  class PackageBranchTarget(_BaseBranchTarget):
>      implements(IBranchTarget)
> @@ -95,6 +102,11 @@
>          """See `IBranchTarget`."""
>          return True
>
> +    @property
> +    def supports_code_imports(self):
> +        """See `IBranchTarget`."""
> +        return False
> +
>      def areBranchesMergeable(self, other_target):
>          """See `IBranchTarget`."""
>          # Branches are mergable into a PackageTarget if the source package
> @@ -182,6 +194,11 @@
>          """See `IBranchTarget`."""
>          return False
>
> +    @property
> +    def supports_code_imports(self):
> +        """See `IBranchTarget`."""
> +        return False
> +
>      def areBranchesMergeable(self, other_target):
>          """See `IBranchTarget`."""
>          return False
> @@ -258,6 +275,11 @@
>          """See `IBranchTarget`."""
>          return True
>
> +    @property
> +    def supports_code_imports(self):
> +        """See `IBranchTarget`."""
> +        return True
> +
>      def areBranchesMergeable(self, other_target):
>          """See `IBranchTarget`."""
>          # Branches are mergable into a PackageTarget if the source package
> @@ -311,6 +333,11 @@
>          """See `IBranchTarget`."""
>          return self.productseries
>
> +    @property
> +    def supports_code_imports(self):
> +        """See `IBranchTarget`."""
> +        return False
> +
>
>  def get_canonical_url_data_for_target(branch_target):
>      """Return the `ICanonicalUrlData` for an `IBranchTarget`."""

> === modified file 'lib/lp/code/model/codeimport.py'
> --- lib/lp/code/model/codeimport.py	2010-03-26 01:44:52 +0000
> +++ lib/lp/code/model/codeimport.py	2010-03-26 01:45:34 +0000
> @@ -225,6 +225,8 @@
>                  review_status = CodeImportReviewStatus.REVIEWED
>              else:
>                  review_status = CodeImportReviewStatus.NEW
> +        if not target.supports_code_imports:
> +            raise AssertionError("%r doesn't support code imports" % target)
>          # Create the branch for the CodeImport.
>          namespace = target.getNamespace(registrant)
>          import_branch = namespace.createBranch(

I think raising AssertionError here is OK.