This gist demonstrates how global and local variables work with mixin content blocks in Sass.

0_content_block_scoping.scss

$global: one;

@mixin foo {
  $local: 1;
  $mixin-local: true;

  global-before-content: $global;
  mixin-local-before-content: $local;

  @content;

  global-after-content: $global;
  mixin-local-after-content: $local;
}

div {
  $local: 2;

  global-before-include: $global;
  div-local-before-include: $local;

  @include foo {
    $local: 3;
    $global: two;

    global-within-content: $global;
    div-local-within-content: $local;

    // The next line would raise an error because
    // $mixin-local was defined in a different scope.
    // mixin-local: $mixin-local;
  }

  global-after-include: $global;
  div-local-after-include: $local;
}

1_output.css

div {
  global-before-include: one;
  div-local-before-include: 2;

  global-before-content: one;
  mixin-local-before-content: 1;

  global-within-content: two;
  div-local-within-content: 3;

  global-after-content: two;
  mixin-local-after-content: 1;

  global-after-include: two;
  div-local-after-include: 3;
}

Thanks for this. Ive been tinkering with ways to redefine variables inside of media queries.

Example https://gist.github.com/3486688