As a thank you to all you intrepid testers, here's early access to the first major new feature based on the new backend -
A grid view for book covers (see attached screenshot).
You can get a build of calibre with it enabled from here:
http://download.calibre-ebook.com/betas/
To use it simply click the little button in the bottom right corner that has a grid icon on it.
You can change the cover sizes and background color via Preferences->Look & Feel->Cover Grid