Luckily additions to CSS4 propose a solution.
Before you question why you would consider CSS4 when working with the current browsers note that support for features sometimes appears quickly when it is really needed. This is such a case...
As blogged by Jason Grigsby here there is support in Safari 6 and Chrome version 21 (the most widely used version since late august 2012) for specifying a set of images when defining background images in CSS4.
background-image: -webkit-image-set(url(assets/test.png) 1x, url(assets/test-hires.png) 2x);
Edited example from James Grigsby's blog.
Browser compatibility is not great yet but currently most retina devices are built by apple. A high portion of those users are likely use Safari 6 or Chrome which solves the problem as long as you remember to use the standard background-image for backwards compatibility everybody else.