Rendition is in public beta, and its insanely fast from the looks of it. Basically it's a liveupdate raytracer bit like FPrime for Lightwave or think of it as a very fast feedback IPR renderer. What I don't understand is how it can be so much faster than Mental Ray. Are mental images slacking? Renders FG insanely fast.
You get two buttons on your Rendering shelf tab. One is to send the scene to rendition, the other is for auto update viewport on.
You can also import mr format .mi files to the Rendition client and render that.
I'm impressed but not had a chance to play with it much or give it hardcore testing yet. Currently it supports Maya, Max and XSI. Beta though keep in mind.