I don’t find the color keyer very clean. The best way I find to key something on a white or black backdrop is to duplicate the layer, desaturate the top layer (either with hue saturation, or shift channels whichever gives you the best result, The black and white effect in CS5 might work well too). Then use the levels effect to get a high contrast silhouette. Finally, set the lower layer to use the top layer as either a luma matte, or inverted luma matte depending on if you started with a white or black backdrop.
Hope that helps,
-Matt