Similar, if not, same problem:
13" Macbook Pro (Early 2011), OSX Mountain Lion (10.8), and 30 Cinema Display (max resolution 2560 x 1600)
My 30" display will only display at the max resolution of 1280 x 800. I tried the above hacks, but they didn't work. I even called apple care, which has 90-day free telephone support for Mountain Lion. Their best person, some idiot 'Mike', could only come up with this, 'There is a problem with your 30" display, take it to an Apple Store'.
There is a coincidence that idiot Mike failed to comprehend or acknowledge: my perfectly good 30" monitor stopped working the minute I upgraded to OSX Mountain Lion. My $3000 monitor, albeit a few years older but still light years ahead of most other products, is not supposed to be viewed in 1280 x 800 resolution.
Apple people, assemble! or something! please help!