Skin shading is very simple using the Standard Surface shader in Arnold 5. From the Arnold user guide:
“When rendering skin, you would use a Radius color value like R:1.0, G:0.35, B:0.2, indicating that red should scatter deepest and green and blue less. This would replace the three layer workflow for skin where you would make the deep layer red to indicate that deep should scatter with a larger radius. Setting scatter_radius.R to a larger value would correspond to that.”
Below is a an example of such a skin shading setup (digital Emily files can be downloaded here).
Arvid Schneider has created a video that goes through the skin shading process: