It's like the iTunes album art screensaver, but for Rdio.
The screensaver is available for download here: https://dl.dropbox.com/u/120686/ArtFlipSaver.saver.zip
You can see what the screensaver will look like (without downloading) by loading the same website that is loaded in the screensaver right here: http://artflip.herokuapp.com
Yep. Modern web browsers support all the fancy album-flipping action so I just let them do the hard work. The screensaver is simply a webview that loads the web app above. If you're interested in the app, you can check out the source code here: https://github.com/endenizen/artflip.
The screensaver currently supports only Mac OS 10.8. If you'd like to create a version of the screensaver that runs on another platform, feel free! All you have to do is find a screensaver that can load a webpage and add some configuration options. Check out the companion project artflip (above) to see how to integrate with the web app options.