xfwm won't fix this, so here's a really simple patch, see :
http://bugzilla.gnome.org/show_bug.cgi?id=541154#c6 i hope that'll be approved...
Index: empathy-0.23.3/libempathy-gtk/empathy-ui-utils.c =================================================================== --- empathy-0.23.3/libempathy-gtk/empathy-ui-utils.c +++ empathy-0.23.3/libempathy-gtk/empathy-ui-utils.c @@ -1240,8 +1240,8 @@ XA_CARDINAL, 32, PropModeReplace, (guchar *)&data, 4); - gtk_window_set_skip_taskbar_hint (window, TRUE); gtk_window_iconify (window); + gtk_window_set_skip_taskbar_hint (window, TRUE); } /* Takes care of moving the window to the current workspace. */