Most of the time its ok to mix maya base shaders with mentalray shaders in the same shader network but sometimes things don't work as you would hope. An example of this is when you layer something like a blinn and a mia_material_x using mib_color_mix. At first glance the render looks correct, but you will find the blinn is missing in the alpha channel. A quick fix is p_color_plus_alpha.
About 6 months ago I wrote about using mib_continue and mip_rayswitch to stop a surface blocking final gather rays. I used this technique regularly yet often wished there was an easier way to do the same thing - and there is!
Alessandro Prodan posted a solution on cgtalk that uses a new attribute called miFinalGatherHide to control whether a surface should block final gather rays.