Rob Rumble came up with this, smart stuff!

robsponsive-mixin.sass

@function size-of($position)
  @return nth($position, 1)

@function locator-of($position)
  @return nth($position, 2)

@function resolution-of($position)
  @return nth($position, 3)

@function px-resolution-of($position)
  $resolution: resolution-of($position)
  @if unit($resolution) == 'em'
    @return $resolution / 1em * 16px
  @else if unit($resolution) == 'rem'
    @return $resolution / 1rem * 16px
  @else if unit($resolution) == 'px'
    @return $resolution
  @else
    @error "'#{unit($resolution)}' is an unsupported unit for resolutions."

// massages data for responsive-core
=responsive($property, $positions...)

  // adds sizes and locators to resolution-only positions
  @for $i from 1 through length($positions)

    $position: nth($positions, $i)
    @if length($position) == 1
      $position: null at nth($position, 1)
      $positions: set-nth($positions, $i, $position)

  // if only has one element, massages accordingly
  @if length($positions) == 1

    $p: nth($positions, 1)
    @if locator-of($p) == 'at'
      $positions: null at 0, $p, null at infinity
    @else if locator-of($p) == 'before'
      $positions: size-of($p) at 0, size-of($p) at resolution-of($p)
    @else if locator-of($p) == 'after'
      $positions: size-of($p) at resolution-of($p), size-of($p) at infinity

  +responsive-core($property, $positions...)


// where the magic happens
=responsive-core($property, $positions...)

  $vw: 100vw

  // handling before-locator positions
  $p: nth($positions, 1)
  @if locator-of($p) == 'before'

    @if not size-of($p)
      @error "before-locator positions must have defined sizes."

    +media(max-width #{resolution-of($p)})
      #{$property}: size-of($p)

  // looping through all consecutive pairs of positions
  @for $i from 1 to length($positions)
    $p1: nth($positions, $i)
    $p2: nth($positions, $i + 1)

    $resolution-dimensions: min-width #{resolution-of($p1)} max-width #{resolution-of($p2)}
    @if resolution-of($p1) == 0
      $resolution-dimensions: max-width #{resolution-of($p2)}
    @if resolution-of($p2) == infinity
      $resolution-dimensions: min-width #{resolution-of($p1)}

    @if resolution-of($p1) > resolution-of($p2)
      @error "Position resolutions must be in non-decreasing order."

    @else if resolution-of($p1) < resolution-of($p2)

      @if not size-of($p1) and not size-of($p2)
        @error "Consecutive positions must have at least one size defined between them."

      @else if size-of($p1) and size-of($p2)

        @if size-of($p1) == size-of($p2)
          +media($resolution-dimensions)
            #{$property}: size-of($p1)

        @else
          $slope: (size-of($p2) - size-of($p1)) / (px-resolution-of($p2) - px-resolution-of($p1)) * $vw
          $intercept: size-of($p1) - ($slope / $vw) * px-resolution-of($p1)

          +media($resolution-dimensions)
            #{$property}: calc(#{$slope} + #{$intercept})

      @else
        $resolution: if(size-of($p1), px-resolution-of($p1), px-resolution-of($p2))
        $size: if(size-of($p1), size-of($p1), size-of($p2))

        +media($resolution-dimensions)
          #{$property}: ($size / $resolution) * $vw

  // handling after-locator positions
  $p: nth($positions, length($positions))
  @if locator-of($p) == 'after'

    @if not size-of($p)
      @error "after-locator positions must have defined sizes."

    +media(min-width #{resolution-of($p)})
      #{$property}: size-of($p)