The best way to report this is to open a ticket [1] which describes
the problem in detail (which is what you've done here). This ensure
that we have a formal way of tracking that the issue exists, when it
is resolved, etc.
For bonus points, if you're able to generate a source code diff that
contains the fix you propose, that will make it easier for us to make
the change.
For double bonus points, if your diff includes a test case that
verifies the problem exists, and that your patch fixes it, you make it
very easy for us to make the change.
Tests for model validation usually live in
tests/modeltests/invalid_models, which is a list of models known to
have model problems, followed by a list of the problems that Django
reports.
[1] http://code.djangoproject.com/simpleticket
Yours,
Russ Magee %-)