You can't change that automatically. You will need to create a subclass
of the model field that overrides the formfield() method to return a
forms.DateField with an appropriate input_formats parameter passed in.
In Django 1.1 there'll be optional support in the i18n framework so that
every locale will use their native date format (d/m/Y pretty much
everywhere except en-US) and that should make this a less common
question.
Regards,
Malcolm