Strauss Shader bug in Render To Texture
You cannot use a material using the Strauss Shader when trying to output to the diffuse map with Render To Texture in 3ds Max 2010. I discovered this when trying to bake a texture into a model. If the material uses the strauss shader or is a material with a sub-material using the Strauss shader... the Render to Texture dialogue will not allow you to choose the diffuse map slot as the output map.
I don't regularly use the Strauss shader... so I don't know if this problem applies to previous versions of 3D Studio. I only happened on this since I was being lazy and applied a material that ships with Max that uses the strauss shader to a model. When I changed the shader from Strauss to Blinn, the diffuse target returned to the render to texture output dialogue.
To me, this seems like a bug. For all I know, it may be some limitation to the strauss shader itself.
2011-11-12 Since writing this, I now know this isn't really a bug. Some shaders simply don't have the same maps as others.