I wrote a script which when executed automatically sets the GNOME wallpaper as BING background of the day. This is a PHP script. To execute this script you need to have php installed on your system. You can install php in Ubuntu as

sudo apt-get install php5 php5-cli

Now you are all set to download this script. Download Link: http://www.spsneo.com/scripts/bing_wallpaper.tar.gz

Download this file, extract the script file. Now you can run the script from terminal :
php bing_wallpaper.php

And your wallpaper is set. You can run this script daily from your terminal or you can set a cron job for the same or you can set up an icon in your gnome panel for this script.

I will keep updating this script. And I am also planning to write the same script in python. So check back again if you dont want to install php on your system.

Send your feedbacks and suggestions.

Update 1: I have modified the script to work with http proxy. Just assure that the environment variable http_proxy is properly set.

Update 2: Modified the script to keep the image centered on the desktop with black background. Try it out.