From Patryk Zawadzki:
Here’s an idea for GNOME 3.x. Instead of showing a static wallpaper, start treating the wallpaper as an infinite plane. Basically instead of using a JPEG or PNG file as input, build a library that given a rectangle returns the image data (raster or even better vector) corresponding to the surface it covers. As monitors and workspaces come and go, the shell can expand and contract the background, calling the library as needed to build the missing parts.
Awesome! And parallax, multiple monitors. Great idea!