- first make the functional field writable (if you don't like it to be writable by the end user, then addtionally you'll need to set it as readonly in view)

in v7.0 it may be something like:

    def _set_my_field(self, cr, uid, ids, field_name, field_value, arg, context=None):
        pass

    'my_field': fields.function(_compute_my_field, fnct_inv=_set_my_field, method=True, string="My Field", type="integer")

- then set default value by the ordinary way as for other fields...