Why would xfce4-panel suddenly not be started at boot/login?

[Chris G]

My wife's xubuntu 9.04 system has somehow lost xfce4-panel when
starting up.  I noticed the last time she shut down it seemed to do
something a little odd on the way down and now when she logs in
there's no xfce4-panel running.

Starting xfce4-panel manually from the command line gets things
working OK but why isn't it getting started automatically in the
normal way?  What starts it?

-- 
Chris Green