today, vino (the gnome vnc-server) killed my background image (at least to me, vino seems to be responsible for this, since it has an option to switch off the backround on existing remote connections). some experimenting with compiz-settings, nautilus and bonobo-activation-server did not show the expected results, but some wizardry via gconf-editor did it:
/desktop/gnome/background/draw_background: yes