LOL who renders in Maya? Kidding, fluids used to have to be rendered in Maya.
I've rendered in Mental Ray and seen the usual MR differences, that being a little bit better calculations on the shadowing and a little more refinement in the quality w/o useing fluid quality. On a whole I haven't seen it necessary to use MR for rending fluids.
I wasn't trying to insist that Pixar was using Maya fluids. I'd imagine you are right, most volex based systems would have a look to them. Also I agree that they aren't rendering in Maya, most likely. I don't know their pipeline more than what people say they have/do. From what I've heard that is one of the reasons I think I'm wronge. I guess it's a moot-point. I just wanted to point out another extremely well produced element of this movie.