If show_desktop is set, then the background capplet skips the initial apply_prefs call on the assumption that nautilus might start later to handle the background. 
But if that does not happen (some people still run nautilus-less sessions...), we never call apply_prefs again.

There are two bugs here:

1) we need to call apply_prefs at login eventually, after nautilus had enough time to start up

2) we need to call apply_prefs when background settings in gconf change. The code tries to do that, but fails. Admittely, gconf_client_notify_add is a bit of a trap, since it won't work without a corresponding gconf_client_add_dir.

The attached patch fixes both of these issues.