I just recently purchased a new 15" MacBook Pro; non-retina though. It's running Mountain Lion. Previously, I had four year-old MBP that only had the displayport, no thunderbolt. I was running a Dell U2410 monitor from that display port connection. It ran perfectly, no issues. I have since hooked up my new MBP to that monitor and the monitor looks like crap! It's still running off of the same displayport cable. The resolution says it's 1920x1200. But it's extremely grainy and washed out. There isn't a firmware update for this monitor that I could find since I'm running on a Mac. Suggestions?
Eric - I have the same problem with my Dell monitor + RMBP, and the issue seems to be that for some reason the newer Macs are recognizing these monitors as TVs and sending out a YPbPr signal instead of RGB. All of the OS's profiles and calibration tools are intended for RGB.
You can verify this if you navigate the settings menu on the monitor. Manually switching to RGB only distorts the signal since it really is receiving YPbPr from the Mac.
Although I haven't tried it the current workaround is to use a TB-to-DVI cable, which evidently causes it to be detected correctly. I have reported this issue to Apple, since I think that should be unnecessary; please be a squeaky wheel and do the same. Hopefully Apple will fix it if they get enough complaints.
I purchased a thunderbolt to DVI adapter and hooked up the monitor through that. It works great now! However... it does not put the monitor to sleep. It will only shut the screen off but full power is still sent to the monitor. Previously with on my old MBP with displayport, when the computer screen shut off, so did the monitor and the monitor was put the sleep. This is annoying because the screen still glows. But at least I've got one thing fixed and I can use the monitor properly. I have sent this to Apple.