I can confirm the same issue. It seems that Peview, Safari, Chrome are affected. When the image is opened in Safari or Chrome and zommed out to 100% the image is twice as big as it was under Mountain Lion - no retina functionality. But when the images are bult in into hi-dpi www pages it works properly.
Maybe it is connected to the changes introduced in the scope of the image displaying on non hi-dpi www pages that are now diplayed with no an ugly blur effect.
I hope this will be fixed soon as it is annoying that the retina is not currently supported in such a base tool like Preview.