When you install the recently released Windows 8 Consumer Preview it comes with two themes pre-installed. One is the origami betta fish and the other one is a nature theme with multiple backgrounds.
Well Microsoft has released a nice fresh theme to go along with the fresh consumer preview and it is called Nightfall and Starlight Panoramic Theme. It contains images that are intended to be stretched across two monitors with matching resolutions however, it can be installed in a single monitor setup as well but only the center portion of each image will be visible. I have it installed on my Acer Iconia Tab W500 and it looks terrific.
The download page also states that this theme can only be installed on the Windows 8 Consumer Preview or the Developer Preview that has been out since last September.
There is no indication on the page why it only works with Windows 8 but I am going to try and find out and I will update this post with what I learn.
Here are the images from the theme: