My Lenovo T420 has an NVS 4200M. In my BIOS I have Optimus disabled and it is set to use discrete graphics. After configuring xorg.conf, X loads up and I can use the computer reasonably well. However, changing the brightness consistently hangs (e.g., using brightness keys on laptop, idle dimming in gnome-power-preferences). The machine is totally unresponsive: VT switching doesn't work, Magic-SysRq doesn't respond, and pressing caps lock does not toggle the keyboard light. It seems like the kernel is locking up.
I've found some other situations that render my computer unresponsive, but they're not always as reproducible.
I am running Linux Mint Debian Edition which follows Debian testing. I have reproduced this on 285.03 (packages from experimental) and 275.09 (testing).
I am currently using TwinView with an external monitor, but it is reproducible with no external displays connected.
I saw another user in a different forum apparently having the same issue: http://forums.nvidia.com/index.php?showtopic=207660