Which is a fair point, but it depends who you're targeting: if affluent users are more likely to have high-DPI devices (iPad 3, most smartphones, high-end MacBooks), and they are, then you're going to have to make your site 'retina-ready.' And it's a mistake to thing this is just a young, tech-savvy issue. How many older people are buying iPads? Do you think they'll notice, subconsciously or not, if your site looks a bit crappy?

I have a hard time believing it would be a significant issue to do a 2x image resize, especially if you provide exact image dimensions in your img tag to start with to avoid the renderer having to wait until the image has downloaded to layout the page properly. In any case, I think someone should do some benchmarks to see what kind of an issue it is in practice.

Either way, it's not a bad thing to be forward thinking with design. The amount of 'retina' devices in the world is growing exponentially. At the moment, it's largely an Apple problem, but with high DPI panels now on the market it'll soon be industry-wide.