I've upgraded from 11.10 to 12.04 and installed Gnome 3.4.
I've selected the default Gnome wallpaper which is supposed to change througouht the day but it does not change...
You may simply be encountering a bug or shortcoming in the wallpaper. As far as I know it only changes in intensity depending on the time of day.
You may not even notice the change in fact.