keyboard input stopped being accepted after login I just disabled my screensaver successfully by entering my password, then after that my machine no longer accepts keyboard input.  The only thing I was able to do is change windows with option-left/right/up/down, but no other keys worked.  The mouse worked.

I have ssh and vnc sessions open.  The VNC session also does not accept keyboard input, so it cannot be a physical problem with the keyboard.  The ssh session does, so the computer is alive and well in some limited sense.

How can I debug this?  Are there any command-line equivalents to KeyboardViewer.app?  Any recommendations short of 'reboot the machine' would be very helpful.

(running 10.6.8 on this machine)
I'd start with any apps or utilities that might intercept USB devices. I once had a forgotten keyboard tool that was used to alter mapping of command and option keys that was not compatible with a system update and caused a similar problem. 

USB Overdrive? Keyboard macros? Virtual machine apps? Any similar things could be capturing the USB input from the keyboard and failing to pass to the OS.