Please read Nathan Ford’s post Standards, not Prescriptions in light of the ideas being tossed around for responsive images in HTML.
If you are looking for a really good article outlining what is going on at the WHATWG right now, I wholly recommend Jeremy Keith’s post Secret src. I found it to be the one with the least amount of vitriol and most amount of information.
Taken together, I really feel like we are putting the cart before the horse right now in trying to nail down how responsive images should be handled in relation to HTML. I believe that the entire industry is firmly in the “let’s experiment and see what we can do” in regards to Responsive Web Design and all of the other emerging technologies.
Nailing down specifics seems premature at best.