Yeah, it's actually pretty cool here is another tutorial about it, and how to use multiple matcaps and masking to get fast SSS type effects and stuff and how to make it out into a diffuse texture. http://cgcookie.com/blender/2012/01/09/combining-and-creating-glsl-matcap-materials/
I don't think it works with psd's, actually you need and addon just to display multiple layers.Once you do have that you can paint / project on spec, diffuse etc and it saves the layers which i guess you have to import them manually in your psd.…
I would also like the point out that while it's a pain to get materials set up and working with the 3d painting tools, that Blender's projection paint tools are pretty shnazzy. Also using matcaps and masking and baking the results out can be great for laying down a foundation for your diffuse maps. Once you get used to it…