Fantastic.
Except I have never seen a web client that handles JPG this way. Not a single one will stop at target resolution, and will continue to load until they have all the bytes of the image. Furthermore, there's plenty of reasons to download the full image size. Perhaps the image needs to resize dynamically after being loaded. Perhaps I rotated my device from portrait to landscape and now I need a larger image to fill the space. There's no pause/resume mechanism in the format to handle this, and the resulting interpolation during a resize effect would look horrendous. Oh, and btw, depending on the image contents, progressive format JPG can actually result in larger file sizes than non-progressive. Lastly, with responsive design, the contents of the image may actually need to be different for different resolutions. For example, embedded text or iconography may not be legible at smaller sizes.
Now solve for GIF and PNG.
<picturefill /> is at least a format-agnostic approach that doesn't require extra implementation on the server side, addresses all the above concerns, and can be implemented on browsers that don't currently support it using a little bit of javascript.