In Windows 7, it was the "betta fish" wallpaper. A nice little joke alluding to the beta copy of Windows 7 it came included with. We can probably expect something similar for Windows 8 when the beta arrives later this month.
In the meantime, something interesting needs to adorn the new Metro-styled Internet Explorer 10. Something that's fancy, something that shows new users around, and a page that's also useful as a search engine.
That would be Bing's welcome page for Windows 8 users. To have a look, point your browser to www.bing.com/?win8start=1. New users to the Windows 8 Consumer Preview will find the second square from the right helpful, though at the moment, the page it points to is not yet ready for public consumption.
Thanks Camlann for the tip!