The idea truncatable bitstreams is fascinating too. I'm not well versed in networking, but wouldn't the latency of a mobile network kill the benefit of this technique? e.g, by the time the server receives the "connection closed" signal, a large part of the extra data would have been sent, no?
For JPEG 2000, the network characteristics are important, but I don't think it would be too bad on a mobile network. Low DPI devices might get a bit "too much", but it wouldn't be a problem - they can just throw it out (or incorporate more detail).
viewport-dpi - the dpi viewport-max - the maximum possible pixel dimension and possibly viewport-current - the pixel dimensions at the time of the request
Decent responsive design should deal with the differing viewport sizes, but it might be nice to get a hint before delivering your page what direction to weight that response in.
Our goal with retina.js was to make it zero-config: no markup changes, no extra element attributes or flags.
Doesn't really do much to help with the overhead of modifying existing markup. Perhaps if you're already using something like image_tag in Rails, a retina_image_tag helper might not be too much of a stretch.