Everything is in the subject : the address book page title isn't translated.
Adding $OUTPUT->set_pagetitle(rcube_label('addressbook')); solves the problem, and doesn't seem to harm. Here's a "patch" :
--- /home/tapi/roundcubemail-0.1-rc2/program/steps/addressbook/func.inc 2007-05-18 16:38:11.000000000 +0200
+++ program/steps/addressbook/func.inc 2007-10-30 02:51:26.000000000 +0100
@@ -22,6 +22,8 @@
require_once('include/rcube_contacts.inc');
require_once('include/rcube_ldap.inc');
+$OUTPUT->set_pagetitle(rcube_label('addressbook'));
+
// instantiate a contacts object according to the given source
if (($source = get_input_value('_source', RCUBE_INPUT_GPC)) && isset($CONFIG['ldap_public'][$source]))
$CONTACTS = new rcube_ldap($CONFIG['ldap_public'][$source]);
