Paulo José posted some very interesting LibreOffice UI mockups at DeviantArt for which he says the main inspiration was the Blender UI:
Please note that these mockups are not official! More mockups @ http://pauloup.deviantart.com/gallery/28216273
Update January 18th: After seing Paulo's mockup, Nathaniel (thanks!) wondered how it would look with the default Ubuntu Ambiance theme/Humanity icons instead of a custom theme and here is the result:
(Image original link: Flickr)
What do you think?
See also: More LibreOffice Mockups: Citrus UI
Thanks to Paulo José for the tip!