Whether or not you think that Bing's as good as Google in the search result stakes, I don't think I've met a single person with anything negative to say about the fantastic photography that Bing uses as a daily backdrop for its website. It was no surprise that they released some of these beautiful images as themes for Windows 7. We had Bing's Best, Bing's Best 2, Bing's Best 3 and Bing's Best: Japan, but what a lot of us were really looking for was a theme that would automatically update and not force us to keep checking for more. (Incidentally, I had downloaded all of those theme packs and just copied all of the image files into a single theme which would rotate my wallpaper through all of them.)
Now there is the Bing Dynamic theme, which promises "two new photos from Bing every week for three months, with this Windows 7 theme that updates automatically through an RSS feed." Personally I'd be happy to receive daily updates (after all that's how often they update the web page, and in more than one locale - it's a different image in the UK and US every day), but this is better than nothing.
You can find the theme either by going directly to its Windows downloads page, or by right-clicking on the Windows 7 desktop, selecting Personalize from the menu and then hitting the "Get more themes online" link. Either way, when you download the theme pack you will be asked if you want to download enclosures, i.e. the image files attached to Bing Dynamic's RSS feed - you do.
The few images that I've seen since I installed the theme this morning are great, so I'm really looking forward to seeing what Bing Dynamic is going to deliver in the future.