The add-in is named Big Screen Photos, and it is a Flickr photo browser and it is written as a WinFX XAML Browser Application (XBAP). The UI in the screenshots posted on the site look similar to the Windows Media Center start menu in Windows Vista (color schemes, vertical and horizontal navigation). It looks like the add-in will be available for download in the near future.
It is exciting to me to see folks already starting to write real Windows Media Center applications for Windows Vista already. I'm looking forward to seeing what other folks come up with now that we've released the beta 2 version of the Windows Media Center SDK for Windows Vista.