I just submitted a proposal for a new meta tag to flag that high-resolution images are available and should be loaded in place of low-resolution ones for users with high-PPI displays (like the new iPhone 4's Retina display).
Please see: http://wiki.whatwg.org/wiki/MetaExtensions#Proposals (Currently, although you can use the min-device-pixel-ratio CSS Media Query to achieve this for background images, no such mechanism exists for images displayed via the <img> tag short of setting a flag in CSS and using image substitution via JavaScript. This new meta tag proposes a JavaScript-free and easy-to-author mechanism to handle the above use case.) I look forward to hearing your thoughts :) All the best, Aral