After upgrading Nvidia driver to 304.60 (on current Debian testing x86_64 with KDE) on my laptop (HP Compaq 8710w which uses Quadro FX 1600M/PCIe/SSE2), the screen became dim, and brightness keys don't work to change it back. (See attached bug report below). For now I'm downgrading the driver back to 304.51 even though it has a memory leak in Xorg in Twinview mode. But the dimness just makes the new driver unusable.
This case seems to be similar somewhat: