Retina displays can lead to all sorts of new things you can do, some of them headache–inducing. Detecting for retina devices or hi–density displays still isn’t a straightforward thing, but this javascript snippet could come handy for a quick hack:

if((window.devicePixelRatio===undefined?1:window.devicePixelRatio)>1)
    // do something here

More details on this technique from Shaun Inman’s post.