About once a month I sit down at my computer and I am presented with a black screen on my early 2008 mac pro with an nvidia 8800gt. I can see my mouse cursor and move it around the screen and here all of my fans running, but the keyboard is unresponsive and I cannot bring up the unlock prompt. Putting the machine to sleep and waking it does not change this behavior and requires a reboot. The keyboard is USB and the mouse is bluetooth. Both the 30" ACD and the keyboard are connected to my box with this KVM switch: http://www.belkin.com/flip/ I have opened up my case when this problem happens and could not see any status lights or anything like that blinking. I have examined the system logs on the console and could not see anything obvious. I do have a lot of the "am I here?" messages (I am not in front of my mac pro so I may be getting the message slightly wrong). One post I found suggested these messages could be the result of using a KVM switch, but I don't know. I have jiggled my monitor cables and they both seem secure. **Edit ** the "am i here" messages are definitely related to my KVM stich software. I stopped the service and I no longer get these messages cluttering up my logs.