A few websites like Apple's are 'retina optimized' and the way Apple implements it, it is a bit inefficient--the web browser downloads the standard size image,

A few websites like Apple's are 'retina optimized' and the way Apple implements it, it is a bit inefficient--the web browser downloads the standard size image, then some js replaces those images with high-res variants. But sites like that are rare, and still don't account for a large amount of bandwidth used, so I vote: FUD.