Django ORM query modelling question

Dear Django list,

after a very good start with Django and it's excellent documentation,
I've come now across a problem that neither my SQL nor my Django
knowledge is sufficient to solve, so I'd like to ask for your kind help.

The (simplified) Django model is

class Erfasst(models.Model):
datum = models.DateField()
info = models.BigIntegerField()

An example for the contents of this model is, pre-sorted by 'datum':

datum info
-----------------------
2011-FEB-01 4 *
2011-FEB-02 4
2011-FEB-03 4
2011-FEB-04 5 *
2011-FEB-05 5
2011-FEB-06 5
2011-FEB-07 5
2011-FEB-08 1 *
2011-FEB-09 1
2011-FEB-10 4 *
2011-FEB-11 4

What I would like to know are the rows, when ordered by 'datum', whose
'info' changed (those marked with a star above).

I tried

Erfasst.objects.values('info').annotate(Min('datum'))

but that doesn't work, because it groups all rows with e.g. 'info'==4
and thus doesn't report the row with (datum: 2011-FEB-10, info: 4).

Erfasst.objects.order_by('datum').values('info').annotate(Min('datum'))

doesn't work either, because it groups the ('datum', 'info') pairs,
which is one row per group.

What is the correct query for the task?

Thank you very much!

Best regards,
Carsten