What can cause a (software) keyboard failure on a unibody macbook? I've got a macbook on which the keyboard has suddenly failed.  

The trackpad works, the power button first, but pressing any other key on the keyboard has NO effect. 

On the other hand. During boot I can do CMD-opt-P-R to reset the EPRAM (this has no effect), but does show me that at least those keys are working during boot.  

What might be happening here?
Are you able to boot into Recovery Mode and test the keyboard there?


  To start your computer from Recovery, restart your Mac and hold down the Command and R keys at startup.


If that works, consider re-installing OS X or at least booting into Safe Mode to disable errant extensions and plugins.