This is for sites with fluid widths or visitors with slow connections.

How about a plugin that detects the browser width and serves appropriate sized images. Not scaled CSS but different resolution images. The different sized images, small medium, large, etc could be generated by wordpress using featured image thumbnails. WordPress can already make those on the fly. Rather than specifying an image size when writing a post the writterr could choose the option “best fit” and the most appropriate scaled image would be shown in the browser. This way small screens get small images and large screens get crisp hi resolution ones.

This could be an awesome bandwidth saver for folks with lots of pictures.