% rhn-applet-gui
/usr/lib/python2.3/site-packages/gtk-2.0/gtk/__init__.py:90:
GtkDeprecationWarning: gtk.mainloop is deprecated, use gtk.main instead
self.warn(message, DeprecationWarning)
The up2date Icon defaults to the notification area to the gnome panel. I am not familiar with fvwm, but if you are able to add a small gnome panel, insert a notifiaction area, and run the rhn-applet-gui. it should work in theory.
Maybe someone with some experience with fvwm could have some insight?
HTH
MC