The way mentalray has been integrated with maya's native shaders is a constant source of curiosity. Some things work the way you expect, some not at all and some need to be tricked into working. In this post I'm going to share the results of my experimentation with maya's envSphere shader in a mentalray context, which led me to the obvious conclusion that IBL is a better choice.
After years of using maya's own software renderer I'm used to doing things in a certain way. The envSphere environment shader is easy to use. I hook up a fileTexture, or a rampTexture and I can then use the place2dTexture node control exactly how they sit within the sphere. For example if I have a 180deg panorama, I will probably want to set the repeatU=2 in the place2dTexture.
The trouble with envSphere is that it doesnt seem to work properly with mentalray shaders. If I connect it to the camera's mentalray environment shader slot maya crashes when I attempt to render.
To the untrained eye, the mentalray equivalent of an envSphere would appear to be mib_lookup_spherical, but it is not so user-friendly, because it forces you to use a mentalrayTexture node and a whole bunch of other nodes to get anything approaching the functionality of the place2dTexture. Too hard! And I could never figure out how to hook up a ramp texture. Often I have simply resorted to mapping my fileTexture onto a big nurbs sphere. Easy!
But I like messing around with mentalray and the hypershader, and today, almost by accident, I discovered that its possible to use envSphere as a mentalray environment shader after all. Rather than connect it directly to the camera, you need to go via a multiplyDivide node (or something similar). Here is a snapshot of the hypershade connections (click image for the full size version).

I guess the multiplyDivide node acts as some kind of data converter. Here's a render I did.

I'm using a fairly lores image, but it should not be that blurry. What I realized is that you need to set the fileTexture filter=0 in the effects tab (dont set the filter type to none; leave it as quadratic of mipmap). Then when I rerendered I got this.

This works pretty well. I've got the convenience of the place3dTexture to rotate the environment and the place2dTexture to set my repeats and to fine tune the offset. Here is one I did with a ramp instead.

My excitement was short lived though. When I turned on final gather I got this.

Dohhh!
Fortunately, there is a solution - Image Based Lighting. You can create a mentalrayIbl node by clicking a button in the Environment tab at the bottom of the render settings window. There is no need to connect it to the camera. Once the IBL node is in your scene mentalray will use it as your environment.
Here is my artifact free render with final gather where I have removed the envSphere from the scene and created an IBL node and connected my ramp to it.

When you create an IBL it defaults to Type=Image File and expects you to give it the name of your image file. But if you want to fine tune your image mapping with a place2dTexture node or you want to use a ramp then you need to change to Type=Texture and connect a maya fileTexture or a ramp texture.
A minor inconvenience is that you cant simply drag-and-drop the texture node onto the texture swatch in the IBL attribute editor. You can try, but no connection gets made. To connect an existing node you need to use the connection editor and connect as shown here.

Notice how the IBL attribute called "texture" in the attribute editor (below) is called "color" in the connection editor (above right) - just to confuse you.

A bonus with the IBL node is that you get a textured preview in the viewport which really helps you line up your reflections. By default the IBL will be scaled very large and you may need to scale it down to see it within your camera's clipping planes.

The IBL can also do light and photon emission. Neither are turned on by default, but they are there if you want them, and you can afford the render time.
Conclusion: Sometimes it is possible get mentalray to do things it probably wasn't meant to do, but for an environment shader, Image Based Lighting is a better choice than envSphere (and forget mib_lookup_spherical unless you like complication).