Hello Pascal,

You can see the request.uid is taking vaule inside the file : /openerp/addons/base/ir/ir_http.py.

There is one method named : _auth_method_public 

    def _auth_method_public(self):
        if not request.session.uid:
            dummy, request.uid = self.pool['ir.model.data'].get_object_reference(request.cr, openerp.SUPERUSER_ID, 'base', 'public_user')
        else:
            request.uid = request.session.uid

Also, Inside website module this method is inherited to manage request.uid according to website configuration.

I am sure this will guide you in right direction.

Thanks.