I do look forward to seeing the daily image Bing has as background on the search page – and was really excited to see “Bing’s Best” as a Win7 theme, I’ve been enjoying some great photos rotating on my desktop…

[Update] You can also add this functionality to WindowsXP

Wallpaper Rotator, from Microsoft's CodePlex, allows to dynamically change the desktop background (similar to Windows 7). 
(Thanks to Blake from The Road to Know Where)

Personalization Gallery - Windows 7 themes, wallpapers, and gadgets - Microsoft Windows