What I really want in Sass: a syntax for doing media queries inside a selector or mixin...

input.sass

// Typical grid column mixin..
=column($percent)
  width: $percent
  float: left

  // Fancy new pseudo-selector syntax for media queries which would let my 
  // column from above switch to 100% when the window got narrower than 600px...
  &@media only screen and (min-width: 600px)
    width: 100%
  
// Apply the mixin to some elements...
#main-content
  +column(70%)

#sidebar
  +column(30%)

output.css

// Output...

#main-content {
  width: 70%;
  float: left;
}

#sidebar {
  width: 30%;
  float: left;
}

@media only screen and (min-width: 600px) {
  #main-content {
    width: 100%;
  }
  #sidebar {
    width: 100%;
  }
}

Doh! Just realized this is actually already possible with Sass. Nevermind!