I’ve been using Maya for many years now and have never encountered this problem.  I have a piece of geo I want to displace.  The MR approximation editor is assigned to this geo using the normal spacial settings I typically use.  I have the renderer set to MR production quality, default light turned off etc.. and...nothing… Nothing renders just the piece of un-displaced geometry.  What has changed in 2012 that I don’t know about?

hmm only thing i can think about in the moment is that you maybe not have checket “alpha is luminance” in your file texture node!? because i do not have any problems in maya2012 with rendering displacement.

I’ve checked and unchecked alpha is luminance and still nothing...I’ve toggled all the color gains and offsets and it doesn’t do anything.  Any other suggestions??

i know thats a helpless answer but it just worked here as expected (2012 sp2)!
currently i have no idea what maybe went wrong on your side. but if you could provide a scene file and a texture i will have a look maybe i see something then!

Do you have displacement override for the division method in Mental Ray? I would recommend using the subdivision override and dump the Displacement one. Plus turn “Feature Displacement” off. Although it would have been a good idea to turn Feature Disp off before applying any sort of displacement approximation to the geo from the get go.