We're working on making iMore support the new iPad's Retina display (and other HiDPI screens when they ship). Here are some links I've been collecting.
It seems like CSS and background-image based things are *fairly* easy and you can use media queries to only serve one image that's the right size for the device.
I think img tags are far harder to get right. I don't think Apple's solution they used for apple.com is ideal, but maybe it's the best option we have?