When you choose disallow postpone it doesn't remember this option after you logout.

This happens because the plugin doesn't safe the state of the variable "allow postpone" in configuration file for after load it.

I looked into the source code from svn trunk and I made a patch for fixes it, but because I don't know how attach a file in this bug report, follow below:

--- panel-plugin/time-out.c.orig        2007-11-25 14:49:41.000000000 -0200
+++ panel-plugin/time-out.c     2007-11-25 14:49:31.000000000 -0200
@@ -728,6 +728,7 @@ time_out_save_settings (TimeOutPlugin *t
           xfce_rc_write_bool_entry (rc, "display-seconds", time_out->display_seconds);
           xfce_rc_write_bool_entry (rc, "display-hours", time_out->display_hours);
           xfce_rc_write_bool_entry (rc, "display-time", time_out->display_time);
+          xfce_rc_write_bool_entry (rc, "allow-postpone", time_out->allow_postpone);
 
           /* Close file handle */
           xfce_rc_close (rc);