UI designer Louie Mantia, whose "Andy's Room" background I use on literally all of my devices, has started upconverting his wallpapers to Retina format. Only a few of them have been done so far, including all of the ones in the slideshow on his home page, but I think he intends to post new ones regularly until they're all updated.

Also, the Fifty Foot Shadows blog has released a pack of very nice-looking photographic wallpapers.