SCSS mixin for cross-browser background-color: rgba(...). You'll need the Ruby extension for convenient color conversion rgba->hex (IE uses ARGB)

cross-browser-background-rgba.scss

@mixin background-rgba($r,$g,$b,$a) {
  // To mimic this in Internet Explorer, you can use the proprietary filter
  // property to create a gradient with the same start and end color, along
  // with an alpha transparency value.

  @if experimental-support-for-microsoft {
    $color: ie_hex($r,$g,$b,$a);
    $value: unquote("progid:DXImageTransform.Microsoft.gradient(GradientType=0,startColorstr=##{$color},endColorstr=##{$color})");

    background-color: transparent\9;
    -ms-filter: $value;
    filter: $value;
    zoom: 1;
  }

  // Good browsers.
  @include experimental(background-color, rgba($r,$g,$b,$a),
    -moz,
    -webkit,
    -o,
    not -ms,
    -khtml,
    official
  );
}

experimental.scss

// Stolen from chriseppstein/compass ;)

// Support for mozilla in experimental css3 properties.
$experimental-support-for-mozilla   : true !default;
// Support for webkit in experimental css3 properties.
$experimental-support-for-webkit    : true !default;
// Support for opera in experimental css3 properties.
$experimental-support-for-opera     : true !default;
// Support for microsoft in experimental css3 properties.
$experimental-support-for-microsoft : true !default;
// Support for khtml in experimental css3 properties.
$experimental-support-for-khtml     : true !default;

// This mixin provides basic support for CSS3 properties and
// their corresponding experimental CSS2 properties when
// the implementations are identical except for the property
// prefix.
@mixin experimental($property, $value,
  $moz      : $experimental-support-for-mozilla,
  $webkit   : $experimental-support-for-webkit,
  $o        : $experimental-support-for-opera,
  $ms       : $experimental-support-for-microsoft,
  $khtml    : $experimental-support-for-khtml,
  $official : true
) {
  @if $moz     and $experimental-support-for-mozilla   {    -moz-#{$property} : $value; }
  @if $webkit  and $experimental-support-for-webkit    { -webkit-#{$property} : $value; }
  @if $o       and $experimental-support-for-opera     {      -o-#{$property} : $value; }
  @if $ms      and $experimental-support-for-microsoft {     -ms-#{$property} : $value; }
  @if $khtml   and $experimental-support-for-khtml     {  -khtml-#{$property} : $value; }
  @if $official                                        {         #{$property} : $value; }
}

// Same as experimental(), but for cases when the property is the same and the value is vendorized
@mixin experimental-value($property, $value,
  $moz      : $experimental-support-for-mozilla,
  $webkit   : $experimental-support-for-webkit,
  $o        : $experimental-support-for-opera,
  $ms       : $experimental-support-for-microsoft,
  $khtml    : $experimental-support-for-khtml,
  $official : true
) {
  @if $moz     and $experimental-support-for-mozilla   { #{$property} :    -moz-#{$value}; }
  @if $webkit  and $experimental-support-for-webkit    { #{$property} : -webkit-#{$value}; }
  @if $o       and $experimental-support-for-opera     { #{$property} :      -o-#{$value}; }
  @if $ms      and $experimental-support-for-microsoft { #{$property} :     -ms-#{$value}; }
  @if $khtml   and $experimental-support-for-khtml     { #{$property} :  -khtml-#{$value}; }
  @if $official                                        { #{$property} :         #{$value}; }
}

ie_hex.rb

module IeHex

  def ie_hex(r,g,b,a)
    assert_type r, :Number
    assert_type g, :Number
    assert_type b, :Number
    assert_type a, :Number

    unless (0..1).include?(a.value)
      raise ArgumentError.new("Alpha channel #{a.value} must be between 0 and 1")
    end

    r = r.value.to_s(16).rjust(2, '0')
    g = g.value.to_s(16).rjust(2, '0')
    b = b.value.to_s(16).rjust(2, '0')
    a = (a.value * 255).to_i.to_s(16).rjust(2, '0')

    Sass::Script::String.new(a + r + g + b)
  end

end

Sass::Script::Functions.send :include, IeHex