I ran into a bit of trouble. I've set up a fairly simple scene of a lightbulb shot to pieces in Maya 2012. For compositing purposes, I put it on a plane with a use background shader (every value of the basic attributes except shadow matte turned all the way to 0) applied to it. Besides that I have a camera set up with the environment set to pure white. FG & raytracing on, rgba output, IBL set up, everything.
But whatever I try, I still get black reflections on my lightbulb. Ugly ones.
When raytracing refractions through a useBackground shader, artifacts may appear on the useBackground shaded surface. To workaround, set the “refractions” attribute of the refracted object’s shader to off.
So it looks like you might have to separate shadows from refractions, and/or choose a method other than the Use Background Shader. Note that with any shader, you can map the 'matte opacity' attribute to build your own alpha channel. Still, the separate render passes might be less trouble.
I'd actually started writing a response about how you just had to increase the number of reflection and refraction rays, but then figured I'd better actually test it... was surprised to discover the same limitation you've run into.
I found somekind of workaround. I wanted my white environment to show on my model, yet still have shadows. So I used the same plane and the same use background shader.
Then I went into the plane's render stats and unchecked the 'visible in reflections' and 'visible in refractions' boxes. Now it's standing in 'the eternal white' having a shadow, but not showing the ground it stands on.