Be careful when copying and pasting color values between swatches in Max Standard Materials and Mental Ray Materials. The Mental Ray materials are expecting colors with an "Alpha" value as well. This option is not in the color picker for swatches in Standard Materials and when a value is copied and then pasted into a swatch for a mr material, it sets the value at "1.0"
That's fine in most cases but if you paste this color value into the "Camera Mapped Background" setting of a mr Matte/Shadow/Reflection material you'll be wondering why the object with this material keeps showing in your alpha channel! Gotta set the color value's "Alpha" setting to 0.0 in this case.