Mid-2012 MBPR, got a new Samsung SA850T, good for 2560x1440. Plugged it up and could get nothing above HDMI. Tried everything prior in this thread to get the monitor recognized, best I could get was 1600x1200.
On the off chance that it was a sub-par cable from Samsung, went out and got a mini-displayPort to displayPort cable for $16. Shutdown the MBPR this a.m., swapped cables (was using thunderbolt->DVI adapter from Apple and the manufacturer cable previously), and restarted.
All is well in the world now, with awesome real estate.
Hardware does matter evidently, and I'm happy to get the issue resolved at a small cost.