main_ie6.scss

// This file's output is customized for ie6 consumption.
// Serve it using conditional comments or something.

$legacy-support-for-ie6: true;

@import main_stylesheet;

main_stylesheet.scss

// This stylesheet writes code for all browsers using @if to targe specific browsers.

$legacy-support-for-ie6: false !default;

@import "compass";

.something {
  // normal styles
  @if not $legacy-support-for-ie6 {
    // Stuff IE6 should not see
  }
  @if $legacy-support-for-ie6 {
    // IE6 specific stuff
  }
}