Flags?

flags.scss

// Contrived example:

@mixin text-shadow($value, !important) {
  text-shadow: $value $important;
}

a {
  @include text-shadow(none) !important;
}

// Another contrived example:

@mixin button(!ie6, !ie7) {
  // some styles

  @if ($ie6) { /* optional styles for ie6 */ }
  @if ($ie7) { /* optional styles for ie7 */ }
}

.button {
  @include button !ie7 !ie6;
}

// Useful for other stuff?