@mixin shadow ($x,$y,$blurDistance,$size,$color,$word:””){
   -moz-box-shadow:    $x $y $blurDistance $size $color unquote($word);
   -webkit-box-shadow: $x $y $blurDistance $size $color unquote($word);
   box-shadow:         $x $y $blurDistance $size $color unquote($word);
}

Simple stuff. The interesting bit is the unquote bit since an empty default variable doesn’t work.