Retina Sprites mixin by Gaya Kessler, modified to allow sprite scaling.

retina-sprites.scss

/*
 * Retina Sprites for Compass
 * by:              Gaya Kessler
 * last update:     03/11/14
 *
 * Usage:
 * 1. create two folders in your image directory (in this case 'icons' and 'icons-2x').
 * 2. adjust the foldernames defined below if you use different names.
 * 3. create sprite images for pixel ratio 1 screens and put them in the first folder.
 * 4. create sprite images for pixel ratio 2 screens and put them in the second folder, use the same filenames.
 * 5. use the sprite-image in your Sass/Scss using: '@include use-sprite(<sprite-name>)'
 */

//first we'll define the folders where the sprites are and their layouts
$icons: sprite-map("icons/*.png", $spacing: 4px);
$icons-2x: sprite-map("icons-2x/*.png", $spacing: 8px);
$spriteUrl: sprite-url($icons);
$spriteUrl-2x: sprite-url($icons-2x);

//Sprite mixin, works perfectly with standard defines
@mixin use-sprite($sprite, $scale: 1) {
    background-image: $spriteUrl;
    background-position: sprite-position($icons, $sprite, 0, 0, true);
    background-repeat: no-repeat;
    overflow: hidden;
    display: block;
    height: image-height(sprite-file($icons, $sprite)) * $scale;
    width: image-width(sprite-file($icons, $sprite)) * $scale;
    background-size: (sprite-width($icons) * $scale) (sprite-height($icons) * $scale);

    @media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 2dppx) {
        background-image: $spriteUrl-2x;
        background-size: ((image-width(sprite-path($icons-2x)) / 2) * $scale) ((image-height(sprite-path($icons-2x)) / 2) * $scale);
        background-position: sprite-position($icons, $sprite, 0, 0, true);
        height: (image-height(sprite-file($icons-2x, $sprite)) / 2) * $scale;
        width: (image-width(sprite-file($icons-2x, $sprite)) / 2) * $scale;
    }
}

Updated this to define the sprite-url() in a variable outside of the mixin, because having it within the mixin causes the sprite to be checked and generated each time the mixin is used.