TheModel._meta.get_field('the_field').get_default()
As of Django 1.9.x you may use:
field = TheModel._meta.get_field('field_name')default_value = field.get_default()
You can get the field like this:
myfield = MyModel._meta.get_field_by_name('field_name')
and the default is just an attribute of the field:
myfield.default