Mixin that generates both a regular and retina sprite (using the Compass Sprite Helpers) and returns the appropriate declarations and media queries.

_retina-sprite.scss

// Mixin that generates both a regular and retina sprite (using the Compass Sprite Helpers) and returns the appropriate declarations and media queries.
//
// $folder - The name of the folder that contains the icons to sprite.
//
// No styleguide reference.

@mixin retina-sprite($folder) {
  $sprites: sprite-map("icons/#{$folder}/*.png"); // Generates a sprite containing every icon in the supplied folder.
  $sprites-2x: sprite-map("icons/#{$folder}@2x/*.png"); // Generates a second sprite containing every icon @2x resolution.

  background-image: sprite-url($sprites);
  background-repeat: no-repeat;
  @include inline-block;
  text-indent: -9999px;
  @each $sprite in sprite-names($sprites) {
    &.#{$sprite} {
      background-position: sprite-position($sprites, $sprite);
      height: image-height(sprite-file($sprites, $sprite));
      width: image-width(sprite-file($sprites, $sprite));
    }
  }
  @media (min-resolution: 2dppx), (min-device-pixel-ratio: 2), (-o-min-device-pixel-ratio: 2/1), (-webkit-min-device-pixel-ratio: 2) {
    background-image: sprite-url($sprites-2x);
    @include background-size(100% auto);
    @each $sprite in sprite-names($sprites-2x) {
      &.#{$sprite} { background-position: nth(sprite-position($sprites-2x, $sprite), 1) nth(sprite-position($sprites-2x, $sprite), 2) / 2; }
    }
  }
}

Thank you for this, I'm using a slightly modified version to have classes like the one generated by Compass' @include all-<sprite>-sprites:

@mixin retina-sprite($folder) {
  $sprites: sprite-map("#{$folder}/*.png");
  $sprites-2x: sprite-map("#{$folder}@2x/*.png");

  @each $sprite in sprite-names($sprites) {
    .#{$folder}-#{$sprite} {
        background-image: sprite-url($sprites);
      background-position: sprite-position($sprites, $sprite);
      height: image-height(sprite-file($sprites, $sprite));
      width: image-width(sprite-file($sprites, $sprite));
    }
  }
  @media (min-resolution: 2dppx), (min-device-pixel-ratio: 2), (-o-min-device-pixel-ratio: 2/1), (-webkit-min-device-pixel-ratio: 2) {
    @each $sprite in sprite-names($sprites-2x) {
      .#{$folder}-#{$sprite} {
        background-image: sprite-url($sprites-2x);
        background-position: nth(sprite-position($sprites-2x, $sprite), 1) nth(sprite-position($sprites-2x, $sprite), 2) / 2;
        @include background-size(100% auto);
      }
    }
  }
}