Since upgrading to Ubuntu 12.10 (with upgraded nVIDIA drivers), I have no longer been able to use 1920x1080 @ 50 Hz. I can run xrandr -r 50 without errors -and the screen turns off/on again, but the screen is still running at 60 Hz afterwards (and xrandr itself confirms this). I could of course (probably) have downgraded the drivers, but I didn't.. I have however just now found a workaround, I can successfully switch to my defined 50 Hz mode with the nvidia-settings program, like this:

nvidia-settings -a CurrentMetaMode=1920x1080_50_0

And to switch back to 60 Hz:

nvidia-settings -a CurrentMetaMode=1920x1080_60_0

This requires that valid modelines are defined with the names 1920x1080_50_0 and 1920x1080_60_0. See 50Hz Display Modes on Linux with nVIDIA Drivers fore more information. My original problem is also reported by someone else:

"When trying to change refresh rate the screen goes blank for a second as it usually does but rate is not changed. Both xrandr and nvidia-settings show the old refresh rate still."

The report got the following response:

"Thanks for reporting this. I can confirm that, at least on my system, the --rate option seems to be ineffective on a recent 304 driver. I've filed NVIDIA bug 1054623. Feel free to use this bug number in future inquiries about this issue."