cross-browser-rgba-mixin.scss

@mixin rgba-background($color, $opacity) {
    background-color: $color;
    background-color: rgba($color, $opacity);
    background-color: transparent\9;
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#{'#'+hex(round($opacity*255)) + '' + hex(red($color)) + '' + hex(green($color)) + '' + hex(blue($color))},endColorstr=#{'#'+hex(round($opacity*255)) + '' + hex(red($color)) + '' + hex(green($color)) + '' + hex(blue($color))});
    zoom: 1;
}

sass-hex.rb

module Sass::Script::Functions
  def hex(decimal)
    Sass::Script::String.new("%02x" % decimal)
  end
end