Using the following command will restore old wallpaper behaviour, until the next time you open Appearance Preferences / Background:
gconftool-2 --set "/desktop/gnome/background/picture_options" --type string "spanned"
Using the following command will restore old wallpaper behaviour, until the next time you open Appearance Preferences / Background:
gconftool-2 --set "/desktop/ gnome/backgroun d/picture_ options" --type string "spanned"