Found out the hard way! http://forums.nvidia.com/index.php?showtopic=196111 From NVidia: "Apple's Mac OS X version 10.6.7 update removes the NVIDIA Quadro 4000 display drivers during the installation process. As a result, you will not be able to boot into Mac OS X 10.6.7 with a Quadro 4000 graphics card installed. The previous NVIDIA Quadro 4000 display driver version 256.01.00f03 is not compatible with Mac OS X version 10.6.7 and so you can not reinstall this driver after installing Mac OS X 10.6.7. A new NVIDIA display driver for the Quadro 4000 graphics card version 256.2.35f05 will be available from the NVIDIA Software Downloads page on March 22nd, 2011 which will install on Mac OS X 10.6.7. Since Mac OS X 10.6.7 will not let you boot into the OS if the graphics card is missing display drivers, you will need to install an older graphics card into your Mac in order to boot int the OS. Once you are in Mac OS X, proceed to download the latest Quadro 4000 Mac drivers from the NVIDIA Software Downloads and install them. After the Quadro 4000 drivers have been installed, you may re-insert the NVIDIA Quadro 4000 back into your Mac Pro."