There's a lot of high expectations regarding the artwork for Karmic Koala (Ubuntu 9.10). Both the Login experience (aka face browser) and Plymouth is planed to land in this release.
Mark Shuttleworth has been talking about moving away from the brown color (which everyone currently associates with Ubuntu). UDS should revival more information on the subject and hopefully also news regarding a new theme (or at least the initial work for it).
Recently Kenneth Wimer posted a message to the Ubuntu-art mailinglist about an extra wallpaper package for Ubuntu 9.10. Although November is still far away, the incoming artwork wiki is already starting to take shape. I submitted some photos from my "Ubuntu Wallpaper Photo Pack" to the Extra-Wallpaper-Package. I hope at least one of them will make it in.
Today I also stumbled across the great artwork by Adrien Pilleboue titled meditating koala.