Compass's color contrast mixin seems to favor dark text because it depends on the HSLA lightness of a color. Selecting between light and dark text based on a lightness threshold is not enough to make proper color contrast decisions.
W3C discusses an algorithm to ensure a color is well contrasted against a background. http://www.w3.org/WAI/ER/WD-AERT/#color-contrast
I've used the algorithm here for finding the difference between a given background color, and specified light, and dark values. The mixin will use the light or dark value that has a greater difference form the background color.
I left the compass "contrasted" mixin commented in the code, un comment to see the difference between contrast choices.