As I was developing a site yesterday one of my logo images had a 200px constraining but needed sufficient data to display well. I ramped up the DPI to a "Retina" display density of 260 px/inch. I then realized that while native system text on these super dense displays will look much better, most web images won't. Most websites are built to be optimized on equal/less than 1080px wide with a image density of around 96 DPI.
I haven't conducted a study on the average pixel density across the web but 96 dpi seems to be a good guess as is the default on many professional image editing tools and there has been a push to maximize speed at the expense of density. While I upped the density data on that specific image for a specific purpose and it looks good on retina displays, I have to think I am one of the few who has executed such retina web image optimizations yet. Basically, until developers up the densities on their textures you won't see an improvement outside of system textures and text while using "retina" dense displays.
Further proof of this is the need for "special" retina enhanced programs on the app store. It's cool that there are many already, but out of the gates publishers and developers will have to specifically optimize websites and programs specifically for super high pixel densities that can be displayed by devices like the new iPad, iPhone 4S, HTC Rezound, Galaxy Nexus, and that expanding list.