I experienced a problem with several models that I've tried to texture in Photoshop CS5. The problem was that I could paint on the UV tile, but I could not paint on the model itself. In addition, in order for my texture to appear, I needed to change 3D/Render Settings to Constant. When I tried painting directly on the model, it brought up an error stating that the current Render mode didn't allow for it.
Well, it turns out that it was a problem with exporting the model from Softimage. When exporting an .obj file, if you uncheck Export Material, you can then paint away directly on the model in Photoshop and you can also set Render Setting to Unlit Texture.
