I saw someone had made a driver that monitored your input and logged you out of the system if it thought it wasn't you behind the keyboard. Don't remember who or where I saw it though. Was a long time ago. Might have been on TV.

This is feasible. You would need to adapt McDaniels automatic file format identification algorithm. Using keyboard timings to build up a finger print.
http://www.forensicswiki.org/w/images/f/f9/Mcdaniel01.pdf

Identification could also be achieved using only word frequencies, much like a Ceaser cyphers can be broken by letter frequency

Spelling mistakes might be a big give away