The proprietary NVIDIA drivers for Linux are built directly from the same source base as the Windows drivers, the only difference being the interface layer between the kernel and the driver. Feature set and performance are identical to the Windows drivers.
I'm not aware of any known critical bugs in the kernel parts. Usually when stuff breaks in the NVidia drivers, it's because of a combination of a kernel update and sloppy package management by the distribution. None of this should be an issue if Valve decides to release some kind of Linux based 'Steam loader' that boots directly to the game itself, or make their own Linux distro with tight control over the kernel and GPU drivers.
I'm not sure how good the ATI linux drivers are these days though.