> Wait, so by default does that mean that images are shown at 200% zoom? Isn't that going to look terrible?
Yup. It's not that terrible in practice, but definitely not optimal.
> Is there any way for a site author to 'opt-out' of this? Maybe something involving a media-dependent stylesheet or something.
Yup. There's semi-official "hi-def" selector for CSS. Lots of sites are using it: http://www.netmagazine.com/tutorials/master-css-pixels-re...